Specified shipping costs when creating listing are different (less) than actual courier booking costs

It is likely to be one of two things. A/ The address with the courier it is going with is zoned rural, but they have selected urban option, (different courier brands have different rural zoning)

B/ Or that it was priced with one of the couriers on Book a courier, but at the time of booking, that courier was not available, so it reverted to the other courier brand which is more expensive. both are comman issues on TM

Thanks for your comment. If it is the (A) scenario then Trademe should alert me to that fact by saying that the address provided is a rural zone but the customer has selected an Urban zone and shipping prices my vary as a result. 

If it is the (B) scenario then that is very troubling indeed if thats the case. It is no fault of the seller or the buyer if the postage charges or options change between listing and winning the auction. This should be covered by Trademe this cost as it was their system that provided the shipping cost for the customer to base their purchase decision on. Alternatively, Trademe should auto-update the shipping costs on the listing to increase them with live tracking of the shipping options so that the listing accurately reflects the shipping costs/options available at the time of winning the auction. 
Lastly and I would hate to even think it possible, but it makes me wonder if there is something fishy going on where Trademe pockets the difference. IE the estimate costs $11.60 at listing but when the auction sells it now says the cost is $18, which could mean that its actually only costing $11.60 but Trademe takes $6.40 as a fee. 

How on earth is this issue not addressed as its probably the biggest issue ive experienced with Trademe thus far. 

You will be able to assess if it is rural address by using this site https://www.addresscheck.co.nz/ . If they have  paid rural and they selected urban shipping, you can contact buyer and ask them either to supply an alternate urban address, or alternately to pay the extra rural fee required. 

With the B scenario, trademe are not providing the courier service, merly providing the platform where buyer and seller interact and so would not be liable for one of the suppliers not being available at a particular time. Your option here is to wait until the cheaper supplier is back supplying bookings, and I belive the 2 courier brands used can switch their services on or off at will at any time (I dont use bookacourier service myself so this is only what I have heard) in any one day

I dont think there is anything fishy going on but these are the reasons I dont use trademe's book a courier. If its the B scenario you can normally just wait half a day or a day to book the courier (sometimes only hours) and the other courier company can become available again.

If its scenario A, you can check their address yourself on addresschecker and if rural ask them to pay the extra fee or supply a non rural address

I asked Trademe directly when I first noticed this a couple or so years ago and I can’t remember their reasoning but they said “the actual shipping cost will always be less than the estimate so you won’t lose money” … as if that justifies the buyer paying more