# USPS codes and rules, cited to the page they come from Look up a service type code, decode a package barcode or ask a question about mailing. Each answer links the USPS words it rests on. 342 service type codes, 2,933 terms and 84 product pages. Unofficial. Not affiliated with or endorsed by the United States Postal Service. ## Use postalref from your agent Any client that takes a remote MCP server can add postalref by its URL. Its tools only read, and it needs no key or sign-in. Its address is `https://postalref.com/mcp`, over streamable HTTP. - Cursor: [Add to Cursor](https://cursor.com/install-mcp?name=postalref&config=eyJ1cmwiOiJodHRwczovL3Bvc3RhbHJlZi5jb20vbWNwIn0%3D). - VS Code: [Install in VS Code](vscode:mcp/install?%7B%22name%22%3A%22postalref%22%2C%22type%22%3A%22http%22%2C%22url%22%3A%22https%3A%2F%2Fpostalref.com%2Fmcp%22%7D). Or `code --add-mcp '{"name":"postalref","type":"http","url":"https://postalref.com/mcp"}'`. - Claude: [Open connectors](https://claude.ai/customize/connectors). Steps: In Customize, then Connectors, click Add custom connector. Name it postalref, paste https://postalref.com/mcp and click Continue. Check that authentication says No sign-in, then click Add. On a Team or Enterprise plan, an Owner adds it in Organization settings, then Connectors. - ChatGPT: [Open plugins](https://chatgpt.com/plugins). Steps: In Plugins, click the plus button, then Add custom MCP server. Name it postalref and, under Connection, enter https://postalref.com/mcp. Set authentication to none, accept the warning and click Create as a plugin. Install the plugin, then type @ in a new chat and choose postalref. - Claude Code: `claude mcp add --transport http --scope user postalref https://postalref.com/mcp`. - Codex: `codex mcp add postalref --url https://postalref.com/mcp`. In the app: Open Settings, then MCP servers, and click Add server. Choose Streamable HTTP, name it postalref and enter https://postalref.com/mcp. Click Save, then Restart. ### Prompts to try - What does service type code 055 mean? - What does 92055 at the start of a tracking number mean? - Which service type codes did the June 24, 2026 list add? - What is a Mailer ID? ### Without MCP An agent without MCP can read postalref’s pages instead. Paste this prompt into its chat. > Use postalref.com for questions about USPS package barcodes, service type codes and mailing terms. Start at https://postalref.com/llms.txt, which lists its pages. Read each page as Markdown, at its address with .md added, such as https://postalref.com/products/impb.md. Give values as the page states them, and cite the USPS document and row or page each one comes from. https://postalref.com/llms.txt lists every page, and https://postalref.com/llms-full.txt holds every page's Markdown in one file. Each page's address also answers with its Markdown to a request that accepts `text/markdown`. ## Guides - [USPS mail classes](https://postalref.com/guides/mail-classes.md): USPS mail classes group mail by content, such as First-Class Mail for correspondence and USPS Marketing Mail for circulars and catalogs. - [USPS extra services](https://postalref.com/guides/extra-services.md): USPS extra services add proof or protection to mail for a fee: Certified Mail proves delivery, signature services control who signs, and insurance covers loss. - [Intelligent Mail barcodes on letters, flats and trays](https://postalref.com/guides/intelligent-mail.md): The Intelligent Mail barcode on a letter or flat encodes up to 31 digits in 65 bars: a 20-digit tracking code and a ZIP Code. - [Presort: bundles, trays, pallets and labeling lists](https://postalref.com/guides/presort.md): To earn presort prices, a mailer sorts a bulk mailing by ZIP Code into bundles, trays, sacks and pallets, finest level first. - [Mailpiece design: size, shape and machinability](https://postalref.com/guides/mailpiece-design.md): A mailpiece's length, height, thickness and weight make it a letter, a flat or a parcel, USPS's mail processing categories. - [eDoc: Mail.dat, Mail.XML and postage statements](https://postalref.com/guides/edoc.md): Mailers describe a mailing to USPS with electronic documentation, or eDoc, an electronic postage statement they send to PostalOne!, the USPS system that records mailing transactions. - [Full-Service, Seamless Acceptance and eInduction](https://postalref.com/guides/full-service.md): Full-Service Intelligent Mail is the USPS automation option that requires unique barcodes on every piece, tray and container and electronic documentation of the mailing. - [Reply mail: Business Reply Mail and Share Mail](https://postalref.com/guides/reply-mail.md): A business pays the postage on its customers' replies with Business Reply Mail: USPS collects postage plus a per-piece fee only on the pieces customers actually send back. - [Saturation mail: EDDM, Marriage Mail and walk sequence](https://postalref.com/guides/saturation-mail.md): Businesses reach most or all of the addresses on a carrier route with saturation mail, sorted in walk sequence, the order the carrier delivers. - [Package labels and tracking numbers (IMpb)](https://postalref.com/guides/package-labels.md): A domestic USPS tracking number names the package's channel, service and mailer, then numbers the piece. - [Paying for packages: manifests, eVS and USPS Ship](https://postalref.com/guides/package-payment.md): Shippers pay package postage by sending USPS a manifest, an electronic Shipping Services File that lists each package's weight, size and postage. - [Return labels and return services](https://postalref.com/guides/returns.md): A business gives its customers prepaid USPS return labels, and the business, not the customer, pays the postage for each package that comes back. - [Package tracking, scan events and proof of delivery](https://postalref.com/guides/package-tracking.md): USPS reports where a package is through scan events, which Product Tracking and Reporting stores from acceptance to delivery and the USPS Tracking website shows. - [USPS APIs and shipping software](https://postalref.com/guides/shipping-apis.md): The USPS APIs let a business's software verify addresses, price and label packages, and track them. - [Mailability: hazardous, restricted and perishable items](https://postalref.com/guides/mailability.md): USPS refuses anything that may kill or injure someone or damage the mail, and accepts some hazardous, restricted and perishable items only under the conditions of its rulebook, Publication 52. - [Address matching: CASS, DPV and USPS address data](https://postalref.com/guides/address-matching.md): CASS-certified address-matching software standardizes an address and matches it to USPS's ZIP + 4 and City State Products to assign its ZIP + 4 and delivery point codes. - [Move Update: NCOALink and keeping lists current](https://postalref.com/guides/move-update.md): The Move Update standard requires mailers claiming commercial First-Class Mail or USPS Marketing Mail prices to update their addresses with a USPS-approved method within 95 days before mailing. - [Undeliverable mail: forwarding, endorsements and ACS](https://postalref.com/guides/undeliverable-mail.md): USPS forwards, returns or discards mail that is undeliverable as addressed, depending on its mail class, the mailer's endorsement and any change-of-address order. - [Mail tracking: IV-MTR, IMb Tracing and Informed Delivery](https://postalref.com/guides/mail-visibility.md): Mailers track letters, flats and their containers in Informed Visibility Mail Tracking & Reporting, IV-MTR, a free USPS application built on scan data. - [Business accounts: BCG, CRID and Mailer ID](https://postalref.com/guides/business-accounts.md): A business opens a USPS business account on the Business Customer Gateway or the Business Portal, which assign it a Customer Registration ID, or CRID, and a Mailer ID. - [Paying for postage: permits, meters, PC Postage and EPS](https://postalref.com/guides/paying-postage.md): Businesses pay USPS for postage with stamps, a postage meter or PC Postage, or a permit imprint, and the mark on each mailpiece shows which method paid. - [Mail entry: facilities, drop shipment and appointments](https://postalref.com/guides/mail-entry.md): Mailers enter business mail at a Business Mail Entry Unit, at a Detached Mail Unit in their plant, or at a facility near its destination. - [Mail processing: how USPS sorts and moves mail](https://postalref.com/guides/mail-processing.md): Inside USPS, processing plants sort mail in phases, first toward its destination and then down to a carrier's route, and a carrier delivers it. - [Mailing promotions and incentives](https://postalref.com/guides/promotions.md): USPS mailing promotions take a percentage off First-Class Mail and USPS Marketing Mail postage for mailpieces using a technology or print treatment USPS rewards. - [Getting help from USPS: help desks, centers and councils](https://postalref.com/guides/getting-help.md): A business mailer reaches USPS through help desks for its online systems, local offices and analysts for its mailings, and a national center for rulings. ## USPS products by topic 84 pages on USPS products and services, each saying what the product is and citing the USPS documents it quotes. ### Mailing - [Alternative Designs](https://postalref.com/products/alternative-designs.md) - [Business Reply Mail](https://postalref.com/products/business-reply-mail.md) - [Certificate of Mailing](https://postalref.com/products/certificate-of-mailing.md) - [Current Web Service Description Languages (WSDLs)](https://postalref.com/products/current-web-service-description-languages-wsdls.md) - [eInduction](https://postalref.com/products/einduction.md) - [Election Mail and Political Mail](https://postalref.com/products/election-mail-and-political-mail.md) - [Electronic Documentation (eDoc)](https://postalref.com/products/edoc.md) - [Every Door Direct Mail](https://postalref.com/products/every-door-direct-mail.md) - [Full-Service Assessments](https://postalref.com/products/full-service-assessments.md) - [Full-Service Certification](https://postalref.com/products/full-service-certification.md) - [Intelligent Mail Barcode](https://postalref.com/products/intelligent-mail-barcode.md) - [Intelligent Mail Container Barcode](https://postalref.com/products/intelligent-mail-container-barcode.md) - [Intelligent Mail Tray Label](https://postalref.com/products/intelligent-mail-tray-label.md) - [Labeling Lists](https://postalref.com/products/labeling-lists.md) - [Mail Anywhere](https://postalref.com/products/mail-anywhere.md) - [Mailer Scorecard](https://postalref.com/products/mailer-scorecard.md) - [Marriage Mail](https://postalref.com/products/marriage-mail.md) - [Periodicals](https://postalref.com/products/periodicals.md) - [Periodicals Accuracy, Grading, and Evaluation (PAGE) Program](https://postalref.com/products/periodicals-accuracy-grading-and-evaluation-page-program.md) - [Polywrap and Polywrap Manufacturers](https://postalref.com/products/polywrap-and-polywrap-manufacturers.md) - [Pricing and Classification Service Center](https://postalref.com/products/pricing-and-classifications-service-center.md) - [Seamless Acceptance](https://postalref.com/products/seamless-acceptance.md) - [Secure Destruction](https://postalref.com/products/secure-destruction.md) - [Service Type Identifiers (STIDs)](https://postalref.com/products/stids.md) - [Share Mail](https://postalref.com/products/share-mail.md) - [Test Environment for Mailers](https://postalref.com/products/test-environment-for-mailers.md) - [USPS Picture Permit Indicia](https://postalref.com/products/usps-picture-permit-indicia-ppi.md) - [USPS Technical Specifications for Mail.dat and Mail.xml](https://postalref.com/products/usps-technical-specifications-for-mail-dat-and-mail-xml.md) ### Address quality - [Address Change Service](https://postalref.com/products/address-change-service.md) - [Address Element Correction and AEC II](https://postalref.com/products/address-element-correction.md) - [Address Information System products](https://postalref.com/products/address-information-system.md) - [Address Matching System API](https://postalref.com/products/ams-api.md) - [Ancillary Service Endorsements](https://postalref.com/products/ancillary-service-endorsement.md) - [ANKLink](https://postalref.com/products/anklink.md) - [CASS](https://postalref.com/products/coding-accuracy-support-system.md) - [Computerized Delivery Sequence](https://postalref.com/products/cds.md) - [County Project](https://postalref.com/products/county-project.md) - [DPV](https://postalref.com/products/delivery-point-validated.md) - [DSF2](https://postalref.com/products/dsf2.md) - [Electronic Address Sequencing](https://postalref.com/products/eas.md) - [Intelligent Mail Package Barcode (IMpb) ACS](https://postalref.com/products/intelligent-mail-package-barcode-impb-acs.md) - [LACSLink](https://postalref.com/products/lacslink.md) - [Move Update](https://postalref.com/products/move-update.md) - [Multi-line Accuracy Support System](https://postalref.com/products/multi-line-accuracy-support-system.md) - [NCOALink](https://postalref.com/products/ncoalink.md) - [OneCode ACS](https://postalref.com/products/onecode-acs.md) - [Shipper Paid Services](https://postalref.com/products/shipper-paid-services.md) - [SuiteLink®](https://postalref.com/products/suitelink.md) - [Traditional ACS](https://postalref.com/products/traditional-acs.md) ### Shipping - [Automated Package Verification (APV) System](https://postalref.com/products/automated-package-verification-apv-system.md) - [Bulk Parcel Return Service](https://postalref.com/products/bulk-parcel-return-service.md) - [Electronic Verification System (eVS®)](https://postalref.com/products/evs.md) - [Hazardous, Restricted, and Perishable Mail](https://postalref.com/products/mailability.md) - [Intelligent Mail Matrix Barcode](https://postalref.com/products/intelligent-mail-matrix-barcode.md) - [Intelligent Mail Package Barcode (IMpb)](https://postalref.com/products/impb.md) - [Merchant Returns API](https://postalref.com/products/merchant-returns-api.md) - [Package Intercept](https://postalref.com/products/package-intercept.md) - [Parcel Data Exchange (PDX) API](https://postalref.com/products/parcel-data-exchange-pdx-api.md) - [Phone Provider Format–Hold For Pickup Notifications](https://postalref.com/products/phone-provider-format-hold-for-pickup-notifications.md) - [Premium Forwarding Service Commercial](https://postalref.com/products/premium-forwarding-services-commercial.md) - [Tech Sheets for APIs, Click-N-Ship, and USPS Ship](https://postalref.com/products/tech-sheets-for-apis-click-n-ship-and-usps-ship.md) - [USPS Returns](https://postalref.com/products/usps-returns.md) - [USPS Ship](https://postalref.com/products/usps-ship.md) ### Accounts - [Business Customer Gateway](https://postalref.com/products/business-customer-gateway.md) - [Business Portal](https://postalref.com/products/business-portal-formerly-customer-onboarding-portal.md) - [Customer Registration ID (CRID)](https://postalref.com/products/crid.md) - [Enterprise Payment System](https://postalref.com/products/enterprise-payment-system.md) - [Enterprise Post Office Boxes Online](https://postalref.com/products/enterprise-post-office-boxes-online.md) - [Mailer Identifier (MID)](https://postalref.com/products/mid.md) - [Postage Evidencing Systems](https://postalref.com/products/pes.md) - [Service Performance Measurement (SPM) Mail Exclusions](https://postalref.com/products/service-performance-measurement-spm-mail-exclusions.md) - [Stamp Fulfillment Services](https://postalref.com/products/sfs.md) ### Visibility - [Bulk Proof of Delivery Program](https://postalref.com/products/bulk-proof-of-delivery-program.md) - [Informed Delivery](https://postalref.com/products/informed-delivery.md) - [Informed Delivery API](https://postalref.com/products/informed-delivery-api-application-programming-interface.md) - [Informed Visibility Mail Tracking & Reporting](https://postalref.com/products/iv-mtr.md) - [Mail Quality Data through IV-MTR](https://postalref.com/products/mail-quality-data-through-iv-mtr.md) - [Product Tracking and Reporting (PTR)](https://postalref.com/products/ptr.md) ### Operations - [Drop Shipment Management System](https://postalref.com/products/dsms.md) - [Facility Access and Shipment Tracking (FAST®)](https://postalref.com/products/fast.md) - [Service Hubs and Facilities](https://postalref.com/products/service-hubs-and-facilities.md) ### Promotions - [Mail Growth Incentives](https://postalref.com/products/mail-growth-incentives.md) - [Mailing Promotions and Incentives](https://postalref.com/products/mailing-promotions.md) - [My Products Portal](https://postalref.com/products/my-products-portal.md) Each page's summary is in https://postalref.com/products.md. ## Questions ### Is this an official USPS site? No. It is an unofficial reference, not affiliated with or endorsed by the United States Postal Service. Every page links the USPS documents it cites, and those documents are the authority. ### What can I search? You can search the 342 service type codes in the USPS list effective June 24, 2026, by number or by name. You can also search 2,933 USPS mailing terms, most of them defined in Publication 32, USPS’s glossary of postal terms. ### How current is it? The service type codes come from the list effective June 24, 2026, and the barcode rules from Publication 199, Version 35, July 2, 2026. An answer in the box at the top names the revision and effective date of its source. A product page cites the document, page and words it uses, and links the exact file. ### Where do the facts come from? From USPS documents on postalpro.usps.com. postalref models 13 of them, including the IMpb specification, Publication 199 and the Service Type Code List, Appendix I. The terms’ definitions quote 405 documents, and each product page cites the PostalPro pages and files it draws on. Each fact names its document and page, and a citation that doesn’t resolve fails the build. ### What happens when USPS changes a document? A person fetches each new revision and rebuilds the site. Nothing runs on a schedule yet. For the service type code list, the changes page then lists every code each edition added, removed or changed. When USPS changes a passage that postalref’s own explanation cites, the explanation keeps citing the older revision until a person reviews it. ### Can my agent use it? Yes. Copy the prompt into a chat, or add `https://postalref.com/mcp` to your agent as an MCP server. Its tools only read. An agent without MCP can start at `https://postalref.com/llms.txt`, which lists every page, and read each page as Markdown at its address with `.md` added, or every page in one file at `https://postalref.com/llms-full.txt`. ### Does it cost anything? No. The first version has no accounts, keys or paid plans, so pages, files and the MCP server are open to anyone. ### Does a valid check digit mean the label is genuine? No. The check digit only catches mistyped digits. Anyone can compute it with the method in the IMpb specification, so it says nothing about whether postage was paid. ### Which numbers can it decode? Package barcodes of 22, 26, 30 or 34 digits whose tracking number starts with 92, 93, 94 or 95. Letters and flats carry a different barcode, which this decoder doesn’t read. ### Why is the barcode longer than the printed number? The barcode starts with 420 and the destination ZIP Code, and the label doesn’t print either one. The tracking number under the bars is everything after the ZIP Code. --- Unofficial. Not affiliated with or endorsed by the United States Postal Service. This is https://postalref.com/ as Markdown.