Etsy's definition of an irregular package:
Irregular Package
All non-rectangular parcels with a combined length and girth no greater than 108", and rolls and tubes up to 26" long.
Link showing tubes, drum shapes that are considered irregular sizes.
https://www.etsy.com/shipping/us/help/package-typesSo if you box is rectangular in shape and uniform in size and 3 dimensional then it is "not" considered an irregular box.
If you select package/thick envelope and enter in the info, then you'll see the same postage rates as USPS.