When I buy a gift card it takes over 30/45 minutes to have it go through and sent to me. In fact getting a invoice for ebooks purchases takes just that long as well. The only time I ever got a email from them within minutes was that 10% off spin to win coupon thing.
I am convinced they use a different credit card processor for gift cards as they always go through differently then ebook purchases on my Card.
It doesn't look like it was an issue on their end other then a issue with your credit card. Anytime you get a message like that don't try it again. Go immediately to your Credit Card issuer's site and see if the transaction has been made if so then wait a few hours to see if the purchase went through or not. If the charge went through but you received nothing from the online retailer then contact their customer service to see if they have more info.
To expect to see an additional $50 credit is well a little out of the ordinary not to mention no one there has that kind of power to do that there. You have to get really high up to get something like that and that can take weeks. Billing is known to take their time when issuing refunds but I doubt even they have the kind of power to give away free money. The best you could hope for is a coupon for $3 off or something like that.
I had similar happen when paying bills. Did I get my money back or compensated? Nope. They gave me credit on next month's bill from the money they took. You learn fairly quickly when it comes to credit cards to be more careful when using them with online scripts and never run a transaction through twice unless you absolutely sure the first didn't go through.