I searched this thread for the word "reuse" and didn't find any hits for my issue, so I figured I'd post it here. Do not reuse USPS Flat Rate boxes if you are not sending flat rate. I recently tried to save the planet by reusing a MFRB to ship some very light 3D printed DB racks. The cost to ship it was like $7 Priority based on weight (purchased online). When I got to the post office, I asked the postmaster whether it would be an issue and she said no and accepted it.
About a week later, PirateShip charged me an extra $8. Apparently you cannot reuse a MFRB if not sending it flat rate. I searched online about this and found conflicting information. Some say that the reason the USPS puts the Flat Rate markings on the inside of the box as well is to prevent people from turning them inside out and using them for a different service class. But I also found documentation on USPS's website that indicates I could have used it for another service class.
Long story short, here's my tip: Do not reuse a flat rate box for any other service type.
About a week later, PirateShip charged me an extra $8. Apparently you cannot reuse a MFRB if not sending it flat rate. I searched online about this and found conflicting information. Some say that the reason the USPS puts the Flat Rate markings on the inside of the box as well is to prevent people from turning them inside out and using them for a different service class. But I also found documentation on USPS's website that indicates I could have used it for another service class.
Long story short, here's my tip: Do not reuse a flat rate box for any other service type.