In the ship window, the Package dropdown lists your own boxes and the carrier's predefined packaging. Pick a carrier package when you ship in that carrier's box or envelope, for example a USPS flat rate box. For carrier packages SKULabs fills in the package dimensions for you.
USPS package types
Letter, Large Envelope, Small Flat Rate Box, Medium Flat Rate Box, Large Flat Rate Box, Flat Rate Envelope, Flat Rate Cardboard Envelope, Legal Flat Rate Envelope, Small Flat Rate Envelope, Flat Rate Padded Envelope, Regional Rate Box A, Regional Rate Box B, Regional Rate Box C and Soft Pack Padded Envelope.
These work with USPS through EasyPost, Shippo and other USPS connections that support them. Discontinued USPS packages were removed from the list.
FedEx package types
Envelope, Box, Small Box, Extra Small Box, Medium Box, Large Box, Extra Large Box, PAK, Tube, 10kg Box and 25kg Box.
The Extra Small Box is 7.25 x 4.69 x 2.5 in. It works with FedEx connected directly and through EasyPost or Shippo.
UPS package types
Letter, Tube, PAK, UPS Express Box, UPS 10KG Box, UPS 25KG Box, Pallet, and Small, Medium and Large Express Box.
Can't find a package type you need? Contact SKULabs support with the carrier and the package name.