The flat rate envelope is the cardboard one. There is one for Priority, and another for Express. The Tyvek envelope, larger than the cardboard one is very strong - good for anything unbreakable, but is not flat rate. It will be charged at the Priority rate for the weight. THat's why you got charged $7.55. The flat rate boxes are great for anything heavy - not a good deal for light things. If you package in your own brown box, the P.O. clerk can give you a choice of Priority vs Standard mail. Standard (parcel post) will be slower, but less expensive. You can add tape, but just to reinforce the seal. The seals have to reach around the items completely. I'm a postmaster, (posting from my very small office). If I can clarify anything, just ask.