How does shipping items via USPS work?

If the item is fairly large, your best bet is to have the item packaged to the Postal Service's standards before you go to the Post Office to ship the item. This will entail secure boxing. If you item is small (will fit in an envelope or one of their small boxes) then you can buy one of them from them or you can buy some packaging from an office supply store. I'm sure you can get a better deal on supplies at one of the large office supply stores than at the Post Office.

if you're selling light weight small items use your own box because usps's "flat rate" boxes rob you blind. when you pack your items make sure the item does not move around (stuff it with news paper or anything) then bring it to the post office and wait at the counter for someone to call you up and just hand them the boxes and wait for them to ask you what service you want to send your box with. then pay and that's it lol