Once you accept the remedy (paying $300 instead of full price which, if they wanted to, they could have done), your case is lost. You should have NOT accepted it, had them note your account you aren't accepting it and taken it and left. Now you are simply at the mercy of what they want to do from a customer service standpoint but (and I'm not saying you will) if you decided to sue them you'd more than likely be SOL.Because I felt a bit "put on the spot" and didn't feel comfortable leaving with shards of glass. As I stated before, I also rely on the device and needed a replacement ASAP for the weekend.
As far as figuring out how to refund me if we get to that point, I have a receipt and they have the receipt number.
I've included the serial number of the cracked device in my further communications and the foot traffic at this store isn't comparable to say, NYC 5th Ave. I would imagine they can track this thing if need be.
Now, if you paid by AMEX (and some other cards as well), you would be covered for the $300 for the first 90 days. Have you checked with your card provider? I KNOW from my own experience AMEX will pay you the $300 as I had a similar problem with a Nikon camera lens.