could be as simple as not being logged into trademe or attempting to answer when your listing has closed,   

most of my items are larger, but still ok for courier service, a few push the boundaries a bit, the 2 couriers i use are nzc and pbt , nzc will , but load the price, pbt quote 1 price and frequently invoice at different rate, so i gave up as it was always a bun fight, and frequently i never made money on the transaction. it was easier to just not send to waiheke and avoid the nonsense which is ok, i state on my listing no courier to islands, but now the auto customised trademe to the buyer  shows freight to waiheke , which can excite them a bit when i say no, but their viewpoint is also cor...

that would depend on the item and service you are using