They all offer free mailers. I guess the only catch is they have to make up the cost somewhere, so the mailing costs are higher than they would be without the "free" boxes. Also, USPS free mailers are only for the Priority classes, so they make money if they convince people to use that instead of First Class or Parcel Post.You seem to know a lot about USPS. I have a question that maybe you can answer. They offer free supplies such as boxes and envelopes. Where's the catch?
This is completely OT, but while I was checking my answer, I found something interesting: Cremated Remains Mailing Products | USPS.com