I can't directly answer the question - and I've only had one overseas sale recently, which was via GSP. Hopefully someone else will answer.

If you like, I can ask various questions which might (or might not) help identify (or a small chance solve) the issue. You say you've have two length conversations with ebay, so a third is always an alternative, to me...

First such question is:

Has (or can) the buyer used the "request total" (or similarly named) option to ask for the total. I think they have to provide the delivery address at that stage, so (in theory) ebay should know it's nothing to do with Simple Delivery.

For 327150488837, what amount did you enter for postage for the buyers country? (either on the item or on tables), how much did they pay? how much did you receive? On the Order page, does it break down the costs? Could the difference be taxes, import fees etc.?