I recently bought ten stamps from a UK auction house. The invoice added £15 for postage, which was disproportionate to the amount I paid for the stamps. Be aware that auction houses tend to add a "default" postage cost, which I always query. I got the cost down to £5, still more than necessary, but £10 saved. The shipment won't be tracked or registered, but for what I paid, that's a risk I'm happy to take.
It's not obligatory to accept their postage charges, send a reply email and query them