I'm very confused. I searched but I can only find store to customer shipping modules and articles.

I want a module that allows customers to buy a shipping label(and stamp) to send stuff to me. If they don't want to buy the shipping label they can choose to go wait in line at the post office/store.

How does this work? Do I need some kind of subscription/contract with USPS?

I've seen this done before on a few sites.