As in previous cases this looks like the difference between city and rural. Did the buyer choose a general quote but give a rural address? You can check that with the company. If that is the case, the buyer needs to pay the higher amount or provide a non-rural address. It would be helpful if the couriers clarified this

Last time this happened to me I found that the buyer was rural but had paid a non-rural price. I contacted them and they provided a different address.

The problem is that the estimate becomes part of a formal offer of sale. Both sides have made a legal agreement which is then breached when the courier company raises the price overnight.

Hi Arthur. They've done that in the past but I didn't know whether it was inevitable!

Usually the courier costs are the same as the original quote. However, I now have an item which was quoted as $9.45, which the buyer has paid, but the cheapest quote is $18.70 (which uses up most of the selling price). The is completely unreasonable. TradeMe's shpping options on the sale page do not specify the courier company, which means I can't go to either NZ Post or Amamex and says "this was your quote" because there is no evidence of who offered it in the first place. If this can't be resolved I'll have to cancel the sale.