Yes, USPS offers a range of free Priority Mail and Priority Mail Express boxes and envelopes. You only pay postage when using them; sometimes flat rate (as in the box being discussed here), and other times by some combination of distance and dimensions/weight.

Flat rate is particularly interesting because the single rate applies, as implied, regardless of weight or distance. I sell a product that is 3 lb and fits into the aforementioned padded flat rate envelope. This means that I can ship it to the farthest reaches of Alaska or Guam for the same $8.45 (USPS commercial rate) as to any other US address.