the advantage with priority is in the flat rate boxes.... if you got something that is heavy, but not larger, you can get a great price... plus is make calculation shipping easier as you know how much it is going to cost regardless of weight (as long as it fits in the box).

I've used prepaid flat rate shipping and a plain brown box. They don't check it...

Yep, it's only 5 cents more, if you really want to save 5 cents by getting it 20 days later, then there's something wrong in the brain.