From USPS News Link:
Sometimes flat-rate envelopes get too stuffed and no longer get the flat rate.
To avoid this problem, USPS is reminding employees that customers should not turn Priority Mail Flat Rate Envelopes into “kangaroo pouches” by overstuffing them — even if the envelope can be sealed.
As postal regulations note, tape may be used on 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.”
In other words: A small bulge is OK, but a fully stuffed kangaroo isn’t.
USPS wants employees to remind customers that manipulating or reconfiguring a Priority Mail Flat Rate Envelope by exceeding its capacity won’t help them save money.
Here’s why: Although these packages can be accepted, they must be weighed and zoned accordingly using the regular Priority Mail price — not the flat-rate charge.
Employees who have questions about flat-rate envelope pricing can email firstname.lastname@example.org.