I agree with Robert. The Post Office offers supplies to their customers, and some require specific use. If you want to send something Priority Mail, then take a box that they provide. If you don't want to use that service: don't. Don't: "take a box, turn it inside out", "strategically place tape", "let the P.O. see me do it", "make sure you cutoff the small writing on the white part", etc.

If you are not happy with the USPS and their prices, please go to other vendors that provide similar services, but don't steal from the USPS - that only raise the prices for everyone else.