This is easy. Take a spare stand, fill it with a range of cards that appeal to a shopper you struggle to get into the business, put it on the lease line so passers-by can see it and watch to see if the stand attracts people. If the stand does not work, change the cards and keep doing this until you see people who would not usually shop with you stop and take a look.
Look at the photo how one of the people in my newsagency set this up. The branding at the top and bottom are key to making a strong and professional statement.
Don’t assume that having a card department is all you need to do to get people to consider purchasing cards from you.