Your items don't seem to be listed with GSP - they are availabale to be posted in the UK only.
Ebay will charge the buyer for onward postage from UK to Mexico based on the value of the item and what the item is.
If you add GSP and the buyer purchases, you only have to prove delivery to the UK hub. Ebay collect the rest of the postage direct from the buyer and cover the item against loss or damage in transit.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)