US POST OFFICE money orders (not MOs from stores, banks, etc that are easily forged). USPS MOs can be validated and CASHED at any US post office. Once you have the cash, you're good. On the flip-side of that the customer has to trust YOU enough to send it...There's your Catch-22.
Are we talking US only? Or other countries? Or between two countries? Are we talking $100 or $1000 or $10,000?