I'm sure there is a set-up available where the postage is automatically added, and the seller did not remember to take care to prevent that when listing that item. If I had to guess what happened, probably it's a business with more than one employee, and the one answering your mail did not know what to do and so tried to defend what happened. If they had 100% positive up to now, I would give them the benefit of a doubt - especially, since you eventually got the postage back, even though they were slow about it and tried to pass the buck.