Assuming it was actually a card with a number linking to a shop list, then YANBU - a discreet little card saying "if you would like to buy us a gift, there is a list of things we would like at X shop" is better than an actual 2 page list printed out.
I do't understand the angst about lists on MN, it's not like they are a new concept, the only new bit is them being at a particular shop and sending out with the invites, parents and grandparents here had lists, but in those days, it was an acutal printed out list that was circulated amongst the family, fine if you all live close together and the Mother of the Bride is organised enough to control the list to avoid multiple gifts, but few people in modern life live in the same town as all their family and friends, and then marry someone who also lives in the same town and all their family live close by.
I can only assume the people who are anti-lists either didn't realise that when they were DCs, lists were being circulated amongst the adults in the family and/or they came frm the sort of family that didn't do lists so you just got a pile of random stuff when you got married. I can see why a list being circulated that isn't directly asked for might seem more polite, but it does cause problems if friends and family on both sides aren't in contact, and few people live like that anymore.
Plus if Debretts says it ok, I can't see how it's seen as rude...