I received same ebay message, it seems you have to fill out papwork when posting items abroad where buyer has paid VAT, if not done correctly, seller may have to pay again.
You can't do this at PO, has to done on Royal mail website or packlink.
TBH. I don't really understand it either.
Ebay will opt you into GSP even if not in it now, so you can either stay or opt out again,
wait until ebay message you again.
