Sounds like they may be doing an average cost of postage on their listings then they are adjusting the cost later once they know the exact postcode of the buyer.

Whether they can do this I don't know, especially when they have listed a price for postage??

I would think the maximum they could charge for postage would be the price they have quoted and if any adjustments were to be made it would be to charge the buyer less as the postage cost has come up less when they have taken into account your postcode. I feel it is the seller’s responsibility to ensure their costs are covered for posting the item where ever they have selected to post whether around Australia or Worldwide.  In the past I have made errors with calculating postage like undercharging for postage to WA, but I have never asked the buyer to pay more once I have quoted a price.  What I have done though, is to refund the difference if postage came up cheaper for the buyer at the post office.

If you know or can find out the size of the packaging, weight and then enter the postcodes in the AP calculator this would give you an idea on the cost. You will also need to take into account that this seller may be charging for handling fees.

Personally I wouldn’t be paying anymore than what is quoted in the listing. They can make an adjustment if they like, but only if the cost comes up less than the quoted price of $17.80. 🙂