Configuring USPS Flat Rate box/envelope methods


USPS Flat rate box methods are often used with merchants that want to show their customers fixed rates while also packing products in pre-determined box sizes. These methods, alongside envelope methods, are box size driven methods. This means in order for USPS to return these methods, you’ll need to have your boxes configured properly and dimensional shipping feature enabled in ShipperHQ.


  1. Ensure that dimensional shipping is enabled from the features section.
  2. In the boxes section, add the desired USPS Flat rate box/envelope.
    • Note that when you define a box on the basic tab, you’ll see “Box Size” drop-down. This will let you choose the flat rate boxes/envelopes which are pre-defined with the proper sizes.
  3. Navigate to the USPS carrier > Basic > and select the desired method you want to use.

From there, you are all set up! USPS will return the selected method when the designated box for that method is used

Keep in mind that USPS has strict box requirements. Meaning, they will not offer any of the flat-rate methods to us if the box used exceeds or doesn’t meet the required dimensions.

Also, note that USPS will offer the USPS Flat rate medium box method IF the large or small boxes are used for packing and sent in the request their minimum-maximum dimensions for that the other two methods so if you opt to hide the medium flat rate method if the flat-rate large or small boxes are packed into than you’ll need to use a box based shipping rule.(same applies for the small/large flat-rate methods, they’ll return if any of the 3 boxes returned – so use the above doc to hide that method for the undesired boxes.)

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