My humble opinion...for $8, you may have made a customer for life. It wasn't going to make or break the bank for you, so I think it was a very nice thing you did. After all, he was still willing to finish the transaction, after realizing his mistake.
On my other ID, I offer combined shipping for $2.50 per each additional piece purchased. I sometimes sell 15 or more items to one customer. I could hold firm and charge them $37.50 extra, but they would go away feeling ripped off and would not become a return customer. I always adjust that shipping way down and the customer feels like they got a nice break and if I am lucky, they come back later and buy more.
Now, if he had said there was no way I am going to pay for it, I would have filed a NPB, blocked him and not looked back. However, it sounds like he was still being reasonable.
~Bonnie