I mail a lot of stuff, and my guess is that the USPS will not allow you to send that box anything other than Priority Mail. If you have some good tape, slit one corner and basically turn it inside out, then tape the corner back together. Then you can do whatever you want to with the box.
The small Flat Rate USPS box MUST be paid at the flat rate. You are not allowed to use it for other than Priority Mail. If you need to send something media mail, get a different kind of packaging.
