You have to remember that Coys' are not catering to the average savvy collector on this forum but rather well-heeled and successful punters who, on the same day, may be dropping tens or hundreds of thousands of pounds on buying a vintage car. Why would any such person be bothered to trawl the internet for a better price for a poster if it takes them minutes (or seconds) at work to make and cover the additional cost? The bonus is that they can take the poster home with them the same day.
Mark