What it does

USPS retired its Web Tools API on 25 January 2026. The built-in USPS module of CS-Cart 4.17 and 4.18 still calls Web Tools, so those stores no longer get USPS rates, and no CS-Cart version prints USPS labels. This add-on talks to the new USPS APIs (OAuth 2.0) directly with your own USPS credentials: no aggregator account and no per-label fee, you pay USPS postage only. For US stores and marketplaces on CS-Cart or Multi-Vendor 4.17.2 to 4.21.x.

How it works

Create an app on the USPS developer portal and paste its Consumer Key and Secret; for labels also enter the CRID, MID and payment account that USPS gives you. Add shipping methods with Real-time rates and pick a USPS service, markup, free shipping threshold, box and fallback rate. At checkout USPS prices and delivery days appear live. On the order's USPS tab get a rate, buy the label and print it. Void or refund unused labels. In Multi-Vendor, vendors can buy labels with their own USPS account.

Features

Live USPS rates from the new USPS APIs (Shipping Options 3, OAuth 2.0), no Web Tools
Works on CS-Cart 4.17 and 4.18 where the built-in USPS rates stopped, and next to the 4.21 USPS module
Domestic services: Ground Advantage, Priority Mail, Priority Mail Express, Media, Library and Bound Printed Matter
Flat Rate Envelopes and Small, Medium and Large Flat Rate Boxes
International: First-Class Package International, Priority Mail International and Express International
Retail, commercial or contract prices, markup, rounding, free shipping threshold and fallback rate
Box presets, automatic box choice, packaging weight and dimensional weight
Domestic and international labels in PDF, ZPL or PNG with customs forms and remembered HS codes
Tracking number added to a new shipment, tracking updates by cron, optional status on delivery
Void unused labels or request a refund
Address validation with suggestions at checkout and on the order page
Multi-Vendor: per-vendor USPS accounts and vendor-panel labels, or marketplace-owner labels

What you get

Package	kv_usps_v3-1.0.0.zip, installed through Add-ons → Manage add-ons
Updates & support	12 months included. Download new versions from your account and install them in Add-ons → Manage add-ons or the Upgrade Center.
License	1 production installation. Staging, dev and localhost are free and unlimited. The add-on keeps working after the update period ends. How licensing works

Compatibility

We list only the builds we install and test on. “Tested” means the full test plan was run on that build; “Supported” means the build is inside the supported range and uses the same hooks.

CS-Cart version and edition compatibility
CS-Cart version	CS-Cart	CS-Cart Ultimate	Multi-Vendor	Multi-Vendor Ultimate
4.22 and later	Testing next	Testing next	Testing next	Testing next
4.21.2	Supported	Supported	Supported	Supported
4.20.x	Supported	Supported	Supported	Supported
4.19.x	Supported	Supported	Supported	Supported
4.18.x	Supported	Supported	Supported	Supported
4.17.2	Tested	Supported	Tested	Supported
Before 4.17.2	Not supported	Not supported	Not supported	Not supported

Last tested build: Multi-Vendor 4.17.2, Nov 5, 2026Tested on: Multi-Vendor 4.17.2, CS-Cart 4.17.2

Requirements

Platform	CS-Cart or CS-Cart Multi-Vendor 4.17.2 – 4.21.x
PHP	PHP 7.4 – 8.3 with the cURL and OpenSSL extensions (standard on CS-Cart hosting). The server must reach `apis.usps.com` (and `apis-tem.usps.com` for the USPS test environment) over HTTPS
Platform	A US Dollar currency in Administration → Currencies (USPS prices are in USD; CS-Cart converts them into your store currency)
Requirement	A USPS developer app (free) for rates, address validation and tracking
Requirement	For labels: a USPS business account with a CRID, a Mailer ID (MID) and an Enterprise Payment Account (EPA), and the Labels and Payments APIs added to your app by USPS
Core changes	None. Hooks and template overrides only

1.0.0

Nov 5, 2026

Latest

Added 10

Real-time shipping service "USPS (new APIs)" on the USPS APIs platform (OAuth 2.0, Shipping Options 3), replacing the retired Web Tools API on CS-Cart 4.17 and 4.18 and working next to the 4.21 USPS module.
21 USPS services: domestic, flat rate envelopes and boxes, international and Global Express Guaranteed (rates).
Per-method retail / commercial / contract prices, markup, rounding, free shipping threshold, fallback rate, delivery estimate, Signature Confirmation.
Box presets, automatic box choice, packaging weight, dimensional weight hint; rate cache shared by all USPS methods.
Domestic and international labels (Labels 3, International Labels 3, Payments 3) in PDF, ZPL or PNG with customs forms and remembered HS codes; shipment with tracking number created automatically.
Void unused labels and request refunds for manifested labels.
Tracking (Tracking 3) on the order and in the customer account, cron updates, optional order status on delivery.
Address validation (Addresses 3) with suggestions at checkout and on the order page.
Multi-Vendor: marketplace-owner or vendor label mode with per-vendor USPS accounts and vendor-panel labels.
Encrypted credentials and tokens, API log with masked secrets, USPS test environment switch.

Tested on Multi-Vendor 4.17.2, CS-Cart 4.17.2. First release.

Why did USPS rates stop working in my store?

USPS retired the Web Tools API on 25 January 2026. CS-Cart 4.17 and 4.18 ship a USPS module that still calls Web Tools, so it returns no prices. This add-on uses the new USPS APIs (OAuth 2.0). Switch your USPS shipping methods to the carrier USPS (new APIs).

I am on CS-Cart 4.21. Do I still need it?

CS-Cart 4.21 includes a USPS module for rates on the new APIs. This add-on adds what it does not have: labels, void and refund, tracking with delivered status, address validation, flat rate boxes per method, box presets, markup, free shipping threshold, fallback rate and per-vendor accounts. Both can run side by side.

What does USPS charge?

The developer app is free. You pay USPS postage for the labels you buy, from your own Enterprise Payment Account. The add-on adds no per-label fee.

Do I need a USPS business account?

For rates, address validation and tracking: only the free developer app. For labels: a CRID, a Mailer ID and an Enterprise Payment Account, and USPS must enable the Labels and Payments APIs for your app (see Installation).

Can I test without buying postage?

Yes. Switch USPS environment to Test: calls go to the USPS Testing Environment for Mailers, where labels are not charged and cannot be mailed. USPS decides which apps get test access.

Which currency are prices in?

USPS prices are in US dollars. Your store needs a USD currency; CS-Cart converts the price into the currency the customer sees. Markup and free shipping thresholds on the Configure tab are in USD.

Why is a method missing at checkout?

USPS did not return that service for the package (for example, over 70 lb, Media Mail for non-eligible items, or an invalid ZIP Code), the USPS account is not set up, or USPS could not be reached. Set a Fallback rate to keep the method visible, and see USPS labels → API log for the reason.

Can vendors print their own labels?

Yes, in Vendors label mode: each vendor connects its own USPS account in the vendor panel and pays its own postage. In Marketplace owner mode the marketplace buys all labels and vendors see the tracking.

Is the customer notified when I buy a label?

The add-on creates the shipment without a notification. Send the shipment e-mail from the order as usual if you want the customer to get the tracking link right away; the customer account always shows USPS tracking.

Where are the labels stored?

In the store database with the order, so they can be printed again at any time. Uninstalling removes them.

Is this an official USPS product?

No. USPS is a trademark of the United States Postal Service. This add-on is not affiliated with or endorsed by USPS; it uses the public USPS APIs with your own credentials.