I think the most confusing thing for people going through the Sell Your Item form is that Cash On Delivery is a default option under the postage/payments tab, but Cash on Pick-up often only appears if you click to show more options (a small link many miss), therefore for most, by simple logical deduction, COD sounds like the one and only option applicable for what is actually COPU...if that makes sense.

To solve this, Cash On Pick Up should be displayed as an option by default, right next to COD, with defining terms so that there is no risk of confusion for buyers/sellers alike.