nope= flat rate envelopes are supposed to be for documents only- more and more post offices are (following the rules) now- we were never supposed to be able to stuff anything in the flat rate envelopes except flat paper-
you can still stuff tyvek envelopes from the post office and the 4 sizes of flat rate boxes- which start at $5.95 to send- those you can put what you want in.
some places the p.o. has always enforced the rules- others were (lax) and pretty much let anything go- with budget cuts and the financial situation of the post office the (big chiefs)have started insisting the rules be followed- so now there are some things we have taken for granted for a few years that will change- just one more of the effects of our present days--