Yeah not much you can do there, only tell the seller how much it's really going to cost and if he says no ask him for a refund for the differnce if your right in the end. The when he sends it you should get a refund for the difference, but then there's always the stubbon losers.
(I just went through all of what I stated above and then the bloke would only refund me $6us not $12us so I took the $6 and neg'd his arse. He wasn't happy and left a lot of crap in my follow up feedback(as he had already gave me +'ve) and then posted a warning about me on beckett but the mods took it straight down
The reason why Ebay are clamping down on this, as sellers list items woth $20 at 99c and charge $20 postage. Sellers still make their money, but only pay FVF on the 99c to Ebay.
So even though Ebay are saying they are doing it "for their members", they are really doing it to improve their bottom line.