From UPS:
<<"Shippers can request that UPS only accept a money order, cashier's check, official bank check, or similar instrument for the exact amount of the C.O.D. package. If shippers do not specify this option, then UPS can also accept a business or personal check. UPS will not accept cash (currency) in any amount for a C.O.D. package. ">>
Fedex:
*We call recipient before delivery so payment can be ready.
*Specify secured or unsecured payment type when preparing the shipping label (if using manual airbills, use the FedEx C.O.D. Airbill).
*Your customer can pay by personal check, money order, cashier's check, company check, official check or certified check.
*Available within the U.S. only.
*Special handling fee applies.