Had this happen to someone the other day, so I wanted to mention this really quick.
They'd paid for chins and shipping, and then decided to come pick up the chins. They would have had their entire shipping refunded, but then they decided to go with an additional chinchilla.
So, I added up what they were getting, and subtracted that from what they'd already paid, and paypalled them a refund for the excess.
Later that day, I got an email asking about if they had been mistaken about the cost of some of the chins, because they thought the refund should have been $50 more than what I'd refunded them. I told them I'd look into it, and when I did, they were correct, they were owed $50 more.
Now, this probably goes without saying, but I'll say it anyway -- I didn't intend to screw that up. I was in a rush when the lady came to get her chins, and was already a little flustered, because that was the same day that I was going out of town and trying to get ready all chicken-with-head-cut-off-style, and I mistakenly wrote down the price of one of the chins wrong.
The reason I write this is to say this -- if you ever think something's not right, I hope you all feel that you can bring it up to me and tell me. I'm not trying to screw anyone out of their money. The prices are what they are, and if I wanted more, I'd just raise the prices. This was just an honest mistake, and I'm glad that she brought it up, as I didn't even realize that I owed her more money.