Documentation //


Postmaster is a platform integrating parcel shipping into your application or website. With one integration you get access to UPS, Fedex, USPS, and Regional Carriers. We support both domestic and international shipping.

Getting Started

It's easy to start shipping with Postmaster, here are the steps:

  1. Signup for an account (free).
  2. Setup your carrier information (like your UPS or Fedex account information, USPS only needs a credit card to get started).
  3. Start shipping (see the API)


Postmaster can initiate a webhook to notify your system whenever the status of a package changes. The message is sent via HTTP POST to your declared URLs.

Adding webhooks:

  1. At navigation bar click at your username. From the dropdown choose Account Settings.
  2. Select Webhooks tab.
  3. Click Add Webhook button.
  4. In form type and URL and click Add Webhook button.

Receiving webhooks:

An HTTP POST message will be send to you with a JSON payload:
Message type; currently, only tracking is supported.
POSIX timestamp of the event.
Content of the response:
Tracking event's name.
Detailed information about the event.
Tracking number.
Related shipment's ID (optional).



What information do I need to start testing?

Nothing, you can test all carriers via your test key with no setup needed.

How long do I have to ask for a refund?

30 days. USPS refunds will take 3-4 days to process and will be rejected if the shipment has already entered the mail-stream.

Can I get a volume discount?

Yes, Pro accounts get access to USPS Commercial Plus pricing and Cubic discounts. Sign-up here.

What label sizes do you support?

We support 4x6 and 4x8 for thermal labels (ZPL, EPL) and 4x6, 4x8 and 8.5x11 for PDF, PNG and GIF.

Can I create sub-accounts?

Yes, please visit the partner API page.

Do you have a question we didn't answer?

Visit the support page.