Problems with family sharing credit card

Having problems with family sharing my credit card. My wife's iphone recognizes her as a family member and shows my credit card as the one shared between us. The problem is, she entered her own credit card information when prompted by the Apple Store. So we've been getting billed separately for purchases. I went in and set her Credit card info to "None" but now she can't buy anything. The store keeps prompting her to enter credit card info it should be picking up mine. How do I fix this?