If the tracking says delivered that is normally an end to the matter. Let it run through resolution, state your case there in a factual, succinct and non emotive manner and off a 0% refund. Your standing along with the buyers will be considered by the SR admins but they would be unlikely to rule against you based upon what you have detailed. I am sure the buyer has a very different version of events though, you will just have to leave it up to the admins to adjudicate if the buyer insists on a refund.