1.4
Flat-Rate Envelopes and Boxes
Any
amount of material may be mailed in a USPS-produced
Priority Mail flat-rate box or flat-rate
envelope. 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.