There's some precedent on how do deal with this on this side of the pond. Require the list price to be achievable with some card at no extra fee (so no cheating with a high annual fee card or some exclusive condition for example) and have the merchant disclose which. Then the extra fee only appears with cards that charge more.

Or put another way: the list price must be reasonably achievable, but charging more for "extras" (eg. a more expensive card with benefits) is always permitted.