You really have no choice - if he has paid you really do need to send it.

Having said that, maybe you could ask them to agree to cancelling the sale, refund the money, you may get a negative, bit not sure how to avoid that. 

Have you looked into how much it is going to cost to post to Mexico?

Did you allow for that when listing ?

If the buyer agrees to the refund and cancellation, before you relist read up on selling, set your restrictions regarding OS etc.

A little more info may help us help you.