Customers might claim that the parcel wasn’t received even though it was marked as delivered - They might tell you that they would file a dispute if they do not get the refund. While avoiding any possible disputes - you may educate customers about the bad effect of disputing the charges.

Talk track: I would not recommend that as that might poorly and negatively affect your credit score especially that it shows as delivered. The first thing that the bank will do is freeze the transaction while they investigate which will block us from issuing any refund to your account. The best thing I can recommend, is you open a claim with USPS so that you could possibly return the the items or I can refund you (1st level - HOLD - 2nd Level)

Note: This talk track will just be your guide to move the conversation forward. Of course, you are permitted to process a full refund if the customer do not want to contact USPS or do not want the partial refund.