It will cost customer money to ship the ammo back. It wasn't the customer's error so customer shouldn't be out money to make things right.
I'd email the seller and advise them of the issue. Further advise that I would be more than willing to return the extra ammo, upon receipt of payment for my time and expenses in dealing with the ammo issue plus an appropriate amount for the return shipping. I would add up my time on phone calls and emails, plus time to pack the ammo for shipping, taking it to the shipper, fuel expenses, etc...
Pretty sure all of those expenses and time would at least equal the cost of the ammo to be returned. If the seller doesn't want to pay my expenses and time, that's on them. Odds are pretty good that I'd be keeping the ammo.🙂
