Hi Amanda,
I'm not sure what you're referring to when you say "Disney merchandise card." I think you mean Disney Gift Card.
Disney Gift Cards can be used in MANY ways including purchases of food at Disney restaurants, merchandise at the shops in the parks and resorts, merchandise at Disney Stores in the US, and even on the Disney Cruise Line.
If you're referring to the Disney rewards cards that are a benefit of the
Disney Visa Card, then this too can be used for purchases of both food and merchandise in the parks and resorts.
If neither of these is correct, then please write back so that we can provide additional help, but if you do mean the Gift Card or Visa Reward Card, then you can definitely buy food at Walt Disney World with the card.