CODs are awkward and inconvenient in the credit card age.
The buyer doesn’t know exactly when the letter carrier will show up with the package, so they might not be home, and often they don’t have the payment on hand and have to accept a notice, then go to the post office to pay for the item and pick it up. Personal checks are acceptable with proper ID. If the buyer wishes to pay cash there’s an extra fee to cover the cost of the money order used to pay the seller.
I delivered maybe one or two COD packages per year in the last five years of my letter carrier carreer. I’ve been retired for four years.