Anyone who sells online has to deal with occasional returns, or disgruntled customers.

As someone else has suggested, figure out what the most efficient practical solution is for you.

If you get upset about it, especially for a prolonged period of time, you will lose emotionally as well as financially.

Just treat it as a practical matter rather than a moral one and you will find it's not a big deal at all. You might even say it's a trivial issue not worth getting worked up at all about. But, the choice is yours. You can get as angry as you want. You will find that doing so doesn't help matters in the slightest, however.