Ken, personally I'd just replace it and I'd not ask for the $50. The reasoning behind it is that the wrong bag was sent out. Your customer has already had the inconvenience of having to send it back to you - time and effort that they'd not have had if the correct one was delivered first time around.
Now, $50 may, or may not be a lot of money to them, but it's money they shouldn't have to spend. They'd not have done this intentionally and I'm sure losing another $50 won't ruin your business. If they do accept having to pay $50 for you to send the replacement don't expect repeat business with them. However, if you send a replacement with an apology then they may say 'hey these guys are great to deal with and have corrected their small error without any fuss even though I [censored] up too'. You maybe able to use it to your advantage.
Just my opinion, and as they say - opinions are like assholes, everyone has one.