I asked the Postmaster (not a clerk) at the Loxahatchee, Florida, branch of the USPS if I can mail other than what the instructions mention on the "Flat Rate Priority Mail" envelope (correspondence and documents), and her response was that I can as long as the envelope seals properly. I didn't question what "properly" meant, though.

This is all very good news to us, Archie... Thanks for calling it to our attention.

JB