So part of the reason for the whole envelope situation is that letter envelopes will go through postal sorting machines which will bend the contents so anything that can’t be bent (for example I once needed to mail a forgotten car key to a family member) can’t be sent in a letter envelope.

Usually the solution is padded envelopes, or for certain things there may even be special postage available like USPS’s Bound Printed Media rate for mailing books (which can I add is such a hilariously federal government-grade obtuse way of saying “books!”) but there’s also “non machine” postage rates available too.

Basically boxes are the easy solution but there’s more efficient solutions available if you’re willing to do a very small amount of research. For people packing it can be as simple as “I can quickly toss this in a box and not worry about it further”