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Connect your USPS account and ShipMagic will query real USPS rates at checkout and show them to the customer.
USPS authentication has changed. USPS has moved from its older API to OAuth-based credentials. You need a Client ID and Client Secret from the USPS Developer Portal — an older USPS username and password will not work.

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Getting credentials

USPS issues API credentials through the USPS Developer Portal. Register your business, create an application, and copy the Client ID and Client Secret it generates.

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Setting up the rate

1

Add a rate to your zone

In your rule, click Add rate on the zone you want.
2

Set the calculation parameter to USPS

Choose USPS under the live carrier options.
3

Enter your credentials

Paste the Client ID and Client Secret.
4

Choose a rate schedule

Select which USPS price schedule your account is on. This must match your actual USPS contract, or quotes will be wrong.
5

Select which rates to show

Pick the USPS services to offer — Priority Mail, Ground Advantage, Priority Mail Express, and the various Flat Rate envelopes and boxes.You can also choose to show All rates, the Highest rate only, or the Lowest rate only.
6

Configure packaging

Set Packaging box distribution and define your boxes. See the packing algorithm.
7

Set a discount and default rate

Discount (Optional) reduces every returned rate by a percentage. Default rate is used if USPS doesn’t respond.

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Flat Rate services

USPS Flat Rate envelopes and boxes price by container rather than by weight, which makes them attractive for dense items. ShipMagic lists them individually so you can offer only the ones you actually stock.
Only enable Flat Rate options you keep in your packing area. Quoting a Priority Mail Medium Flat Rate Box you don’t have leads to a repack at a different price.

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The Discount field

Discount (Optional) applies a flat percentage reduction to every rate USPS returns. Use it to pass on a commercial-pricing advantage, or to absorb part of the shipping cost as a promotion. It does not reflect any actual USPS discount — it’s applied by ShipMagic after the quote comes back.

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Always set a Default rate

If USPS is slow or unavailable and no default rate is set, the customer sees no shipping option at all and cannot complete checkout.Set a default rate that is safe to charge when the live quote is unavailable.

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Testing

Live rates depend on a real origin and destination, so test with complete, realistic addresses.
  1. Add products to the cart with accurate weights and dimensions
  2. Check out to a real US address
  3. Compare the quoted rate against what USPS charges you for the same parcel
Use a fresh address each time — Shopify caches rates per address, and a cached quote will mask your changes.

Packing algorithm

How items are distributed into parcels before quoting.

Adding dimensions

Dimension data that carrier quotes depend on.

Rate blending

Combining carrier rates with your own.

Rates not showing

When a carrier rate fails to appear.