If you just put a pin in an envelope with 1 stamp, it will most likely either get crushed going through the sorting machine, or be caught by a post office worker, stamped 'owe postage' on it, and the buyer has to pay the extra postage in order to get the pin out of the post office. This has happened to me dozens of times. Most of the time it's crushed and useless. Some post office workers are oblivious to this, and sometimes it's the sellers just putting them in their mailbox assuming they will get to the buyer safe and sound.