I see one in every town, but I dont know what they are there for. I am well into the game, I must have missed something. Do you put the $100 NCR, $20 NCR, $10 NCR items you loot from bodies in there and they send you 100, 20, or 10 dollars?

The Mojave Express mailboxes? I think you can use them to send stuff between towns so you don't have to carry it; you just pick it up at the mailbox in the town you sent it to. However I haven't tested that yet since I haven't made it out of Goodsprings yet.

Same EXACT situation and thought process here. lol

They could be used for so much more though. I found a package on a dead guy in goodsprings i think, it said to deliver it to north vegas strip by way of freeside. There is a guy in freeside somewhere I just cant find him. I thought it had something to do with those boxes.