Do they give you 15% on the credit you use or on anything you buy up to $500? If you trade in something and they give you $100 for it, does it mean $115 in stored credit, meaning if I put it towards a $200, I have to cover $85 out of my pocket? Or I'll use the $100 store credit, and the item will be $170, meaning $70 out of my pocket? I could not figure it out, so if a TBer who works at GC or someone who already used this deal knows the answer, that would be great. Thanks!