Not for nothing, but why was it fine at $5...but a big deal at $15? I understand the spirit of the site, but still.
Also, though I really don't know the policy with this, I would speculate that they don't accept returns on the Edge/GI subscription. I mean, if they did, what would stop people from going in, buying or trading in a ton of used games, buying an Edge card, getting all those discounts, and then returning the card?
Lastly, if you do end up being stuck with it, you should take advantage of it. When GS has 25% deals of B2G1, you can certainly save a lot via the Edge Card, especially if you get some of the newest games out. More often than not, they are in near mint condition. I have recently picked up Devil May Cry 4 and Lost Odyssey. Both looked like they were just opened.