Yes you can connect a card to your magic band, though if you're staying onsite the way it works is you essentially run up a 'tab' and once you hit $500 they charge your card or at the end of your stay, whatever comes first. Some vendors such as those that sell glow mickey ears and balloons only take cash last time I checked. It's always good to have a back up on you besides the magic band such as a credit card, cash, or disney gift card as technology is prone to bugs and your magic band has a small chance of failing.