I got sick and tired of having a dozen little "bonus" cards to carry around, so I made this simple fix. Basically, I just scanned in all my barcodes, fit them nicely into a row in photoshop and then printed them out onto a card. There are more detailed instructions that follow if you need them. This page shows the finished product.
Step 1: Scan your barcodes
Get all the cards you have with barcodes, line them up on a scanner and scan into the computer. I used grocery store bonus cards, gas station card, bookstore bonus card, library card, and student ID (of course I blurred them for this instructable just to be safe).
Step 2: Line up barcodes in photo editing program
I had some 'key chain' cards lined up on one side and regular ones on the other to fit them best.