I have an 'email' from USPS stating that any and all FRE's that can close and seal and the envelope is not cut to shape it MUST be accepted..
IF IT FITS IT SHIPS.
I do all my swaps this way.. and I host a swap every month.. I will tape the flap down then I will tape all around front to back to make a 't' with the packing tape.. you will see when it arrives to you..