Can a seller adjust the Ping amount to reflect the actual cost of shipping (after the auction ended) at the request of the buyer?

I have just won an item on Trademe but unfortunately postage was not known at the time (an approximate courier fee was quoted). The Ping amount was for the Auction item alone.

To make payments easier in the future, is it possible to allow the vendor (or buyer) to add the actual cost of postage to the purchase amount after the auction has ended? I certainly understand it would be difficult for the vendor to quote accurate postage costs when yhey dont lnow where the winning bid will come from.

In my current situation (living in Aus), I cannot pay with Ping because there is only my winning bid, without postage. I have the vendors ANZ bank details and went to my loca ANZ branch to try to pay but they dont do overseas transactions. I have tried sending my debit card numbers to the vendor but they dont know how to process the payment (I wouldnt know myself).

Thankfully I have Sis to rely on, still living in NZ. 

It shouldnt have to come to this; there should be a way where the buyer can pay the win price and then add the actual postage costs when they are known. Just my two bob's worth.