I think the local post offices carry TWO lines of shipping boxes and materials.
The NON PRIORITY MATERIALS have to be paid for before using.
The post office only carries SOME of the Priority Mail mailing materials - the rest have to be ordered on-line. They have flat rate, pre-paid, and non-flat-rate materials available. One needs to look carefully at what one is ordering/picking up.
The Priority Mail containers can only be mailed at Priority Mail rates.
Some people were turning the boxes inside out and using them as "ordinary" mail. USPS didn't like that.