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I made this mistake recently and sent a record to the customer before the most recent one. Largely because of a change in the way Paypal works combined with being busy. I also sent a £100 record to Germany by mistake once. I wouldn't call it stupidity but if you've never done the job you might think it is.
In my case as the seller no one else ends up out of pocket. I did receive the records back and paid for the postage and was just happy that punters would take the time to go to the post office to help me out.
Hopefully when you contact them they will do the right thing.
A Discogs seller has sent me the wrong record, meaning someone else has got the tune I ordered.
I can see no way to make this worth my while to sort out for a £5 record and as the guy has sold 13 records ever he's not exactly up to his eyeballs in orders so can only assume stupid which is not likely to make process fun either.