The enlightened firms keep the post and packaging in-house and only charge a reasonable rate for their services, whereas those with less customer focus either outsource this service and abandon their customers to a third party or load the costs to increase their own margins.
In the cycle of buying and selling some auction houses seem to have forgotten that dealers, large or small, need a margin too!
I have learned to check the shipping policy of the auction house before I register to bid and then either adjust my maximum bid or else not bid at all. I suspect that I am not alone.
Recently, I won a lot for a bid of £30 and paid a further £13.49 in P&P (plus VAT) for a small second-hand box, two sheets of The Sun and a postal cost of £4.55, bringing the total cost to me to £52.67. Including the buyer’s premium, I estimate the total margin on my single lot for the auction house to be around £15 or 50% of my bid.
I am hoping that you use this letter to flag this practice and bring it to an end by educating some auction houses about the likely impact of their charges on their customers: they will lose them.