: You can also order these supplies by calling the USPS Package Supply Center at 1-800-610-8734 . 2. FedEx and UPS

: You can find boxes for UPS Simple Rate at The UPS Store . Unlike USPS, UPS often allows you to use your own packaging for their flat-rate service as long as it meets their size requirements. 3. Third-Party Retailers

While they operate differently than the USPS, they also offer fixed-rate shipping options with dedicated packaging.

: For larger quantities (packs of 10 or 25), you can order them for free at the USPS Store . They will be delivered directly to your door at no cost.

: Retailers like Staples and Office Depot often carry flat rate boxes. They may provide them for free if you purchase shipping services through them, or they may sell them as part of their standard inventory.

: Independent stores like PostNet or Mail Boxes Etc. typically stock various flat rate options and can help with packing.