Originally Posted by frugalfabrics
Page 19 of that document - Flat Rate Quick Reference says....
Note: Flat Rate prices are available when material is sent in a USPS-produced Flat Rate Envelope or Box. When sealing a Flat Rate Box or Flat Rate Envelope, the container flaps must be able to close within the normal folds.
Tape may be applied to the flaps and seams to reinforce the container; provided the design of the container is not
enlarged by opening the sides and the container is not reconstructed in any way.
see this is what I have always been told. It has to maintain the original shape and integrity and the tape is just there to keep the lip from getting caught in the machines...or those stupid corners it makes...ugh...I wish the tyvek envelopes would have become the flat rate envelopes...so much easier to ship in those!