
Although the packages can be accepted, they will be weighed and zoned accordingly using the regular Priority Mail price instead of the flat-rate charge.

The USPS is encouraging employees to remind customers that manipulating or reconfiguring a Priority Mail Flat Rate Envelope by exceeding its capacity won’t help them save money. Postal Service® is reminding holiday shippers to resist the urge to turn Priority Mail Flat Rate envelopes into “kangaroo pouches” by overstuffing them – even if the envelope can be sealed. It may be very tempting during the holidays to overstuff your Priority Mail® envelopes, as after all the Post Office™ says “if it fits it ships”, right? That is quite a bit of a stretch of the Priority Mail catchphrase.
