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.
Step 3: Trouble Shooting
My original plan was to print each side in the center of a piece of card stock paper on opposite sides, then just cut it down to size. What printed out was much too large (because of the pixels per inch used in the scanning process). I shrank them down using a copier and ended up just putting the two sides (from different pieces of paper) into a laminator - no need for card stock and no worry about the barcodes smudging over time. If you print out on cardstock, you may want to consider putting scotch tape over each barcode to avoid smudges. I've done this in the past on actual cards and it helps.
Barcodes can be enlarged or shrunk down without it affecting the ability to scan them, so don't worry about distortion. As long as all the bars in the code are in correct proportion, you're good to go.
It would be helpful to cover the barcodes your not using with your fingers so that the scanner doesn't get confused. I will likely go through self scanning checkouts when possible to avoid clerks from freaking out and calling the manager to ask if it's ok to use my card.
Again, below, is the finished product. And I already tried it out at the grocery store and it worked.