: 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.

: You can find flat rate boxes for sale on Amazon and eBay, though these are often resold or specialized versions.

If you need specific types of boxes or want to buy in bulk from other sources:

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

: 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