Hi Archie - Several years ago the USPS introduced an International Priority envelope (flat rate) in two sizes. I was using them to send chips to Europe, and was shortly advised of my missuse because the contents were not "correspondence" or "documents."

I notice the two key words on the new Priority Mail Flat Rate envelope as well. May mean just that: correspondence and documents... and, if so, it's not very clear that only paper can be mailed.

JB