It is no good for big thick heavy things like books.

The crummy cardboard flat rate envelopes are made of recycled material that falls apart under the slightest stress and that dissolves into mush if it gets even a little bit moist. Those envelopes may be good for some things (like physical original patents or physical original trademark certificates) but they are no good for books.

My very strong view is that for the Paris books and the PCT Forms books we should not be using the crummy cardboard priority mail flat rate envelopes (product EP14F). There are the padded envelopes (EP14PE) and there are the crummy cardboard envelopes (EP14F). There are also two kinds of letter size Priority Mail flat rate envelopes. These are not the same thing and you have to pay a lot more postage for the Priority Mail Express as compared to the ordinary Priority Mail.

padded flat rate Priority Mail Express envelopes.

padded flat rate Priority Mail envelopes, and.

There are two kinds of flat rate USPS padded envelopes:

Usps padded flat rate envelope size