In my opinion, it depends on the Auction description
The seller has appropriately set out in advance that international post is $18
Now if he has said - International Post - Glocal Priority, and then he sent via surface mail, well you have a right to be upset and you should leave appropriate feedback
But if he has just said international post $18, and hasn't said how the item will be posted, then I think the onus is on the buyer to make some inquiries about that
You've won the item factoring in a postage cost of $18, so I don't think it is usually appropriate to leave poor feedback later unless the seller has not posted it how he said he would
That's my two cents - but I suspect you will get differing views on this one!