The interior space works out to be absolutely perfect for holding a credit card, and many places have little advertising cards that are this size. For example, the Coca-Cola card from last summer's promotion is this size, and the local Phillip's 66 station has stacks of little cards in this size promoting their REAL credit cards. Many long-distance cards are this size, as well; you can buy them at Wal-Mart and the drugstore.
I have one of these on either side of my RetroStation, and I must say, they DO look nice.