Support setting whether products and packaging is rigid
USPS requires Large Envelopes (Flats) to be "Flexible". See https://pe.usps.com/text/qsg300/Q201c.htm.
Currently, there's no way to specify whether a product or box size meets this criteria, meaning that non-flexible products are sometimes quoted the Large Envelope rate even though they aren't eligible for it. This results in a shipping charge that's too low.
Ideally, there would be a way to specify on each product whether it meets the USPS flexibility requirements. There would also need to be a way to specify this on the packaging, as small boxes would also be the right size but not eligible for the large envelope rate.