I think I tracked down your listing (thatpooch?). You do show only US shipping.

- Don't worry about sending an invoice - one has automatically been sent to the winning bidder by eBay
- don't offer it to the second highest bidder at this point. Wait and see if the Mexico bidder still pays and has it delivered to a US address. It is not unusual for foreign bidders to have a US friend or relative to send items to.
- if he wants it sent to Mexico, decline based on your listing details. If he wants to cancel the transaction, do it through eBay's cancellation procedures so you are protected from a later claim that you breached the auction contract. Only after it has been canceled can you safely offer it to the next highest bidder. You have at least 30 days to make those arrangements, so there is no need to hurry.