When I was offering international postage, unless it was a light item going in a small padded bag, I would send the buyer a message letting them know I would send the invoice after I had checked the postage cost with the post office. Most buyers wouldn't agree to pay the extra after you got the true cost, because they only have to pay the amount that you quoted on the invoice. There is a reasonable chance you will cop a negative for this.

If you didn't want to post the item, you could always tell the buyer that after doing some research, you have discovered that the item in question can't be sent to their country and you would be more than happy to offer a full refund if they have already paid and ask them if they would accept a cancellation request so you can recoup your fees.

I would then go and adjust your settings and block the countries you don't want to post to.