I really didn't know where to post this question, so I will try here

When you place an order with an op and you choose COD delivery via UPS, does it have to be cahs, or can you use a money order? I've never bought anything COD before, and am expecting an order soon but may not be here when it arrives. So I was considering leaving a money order in an envelope taped to my door, w/ instructions for the UPS to leave my package inside my storm door.
If anyone knows if I can use a money order for COD orders, please let me know.
Thanks