Most PO will cash any USPS MO unless they don't have the cash and then might tell you to wait till they do. They can verify authenticity while you wait. Rather than a FTF deal with an unknown, I recc using an FFL Dealer. Their charge is usually minimal and they can hold gun while money order clears. Eliminates any safety risk and they are a licensed entity that can be tracked at least somewhat. Forgeries of cashier checks and USPS MO's is rare and a fed offense. Banks will usually certify if a cashier's check is legit. My experience as a bank attorney and as an escrow agent.