![Flat rate usps priority mail envelope](https://cdn1.cdnme.se/5447227/9-3/15_64e61dfcddf2b36505b4c7c9.png)
Additional size restrictions apply depending on the type of mailpiece you're sending.Īdditional Charges for Irregular Mailpieces & Envelopes Nonmachinable Envelopes All postcards and envelopes (or flats) must be rectangular, otherwise an additional charge may apply.Maximum weight for First-Class Mail letters is 3.5 oz for large First-Class Mail envelopes, the maximum weight is 13 oz.Large envelopes (flats) start at $1.35.Oversized postcards need letter stamps, which start at $0.66.Standard-sized, rectangular postcard stamps start at $0.51.
![flat rate usps priority mail envelope flat rate usps priority mail envelope](https://www.usps.com/stamp-collecting/assets/images/PPEP14F-01-main-278x214.jpg)
Square, oversized, or unusually shaped envelope stamps start at $1.06.
![flat rate usps priority mail envelope flat rate usps priority mail envelope](https://assets.easypost.com/assets/images/usps_boxes/flat-rate-envelope.c51e232ff669cedcb61438e43c1fafba.jpeg)
![Flat rate usps priority mail envelope](https://cdn1.cdnme.se/5447227/9-3/15_64e61dfcddf2b36505b4c7c9.png)