I’m confused. What does the coin have to do with archiving a series? Seems quite harsh. Have you thought about putting the series up for adoption rather than archiving?
£3 postage seems fair to me. Padded Envelope £0.70-£1.00; Postage £1.50 then petrol to drive to the post office.
People will only buy or bid on ebay if they want to buy the item. If they don’t agree with the cost they don’t have to buy.
We all know that on ebay if it says free postage that they just list the item for a higher price in the first place.