Please help! I have never shipped anything.

I am trying to avoid going to the post office. I'm looking on the USPS website and they can send me the box and then pick up the package to ship out. I'm trying to figure out which box to get and how much shipping is going to be. Someone had recommend the flat rate boxes. The issue I'm having with that is the 12x12x8 box (which is around the size I need, I might even need one a little bigger) the shipping would be $20-something. I don't want to spend that much just to ship it. Also, they mentioned either writing the addresses on the box, or getting a shipping label, and then also the postage. How would this work? Would I just pay online? Then what would I put on the box for postage?

Someone please help me out and walk me through how to do this. I'm totally lost