In my experience of using GSP for a short while, it's expensive and slow and offers very little protection against buyers who open 'not as described' cases.
I do sell overseas - I sent a brooch to Australia today - but I simply list items with a price for Royal Mail International Tracked. Then I buy postage using RM Click and Drop and print the label and customs label from the RM site.
"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)