^ Thanks for the detailed information. I feel a bit odd having these discussions, as I didn't anticipate this when making a simple purchase on the web.
The funny thing is I don't want any extra compensation given to me. I just want them to take $12 from my GMG account instead of from my credit card. If they truly can't issue a $12 credit to my CC, then the next best thing is a $12 credit to my GMG account.
I did peruse the chargeback rules you posted earlier, but perhaps if this goes that route I'll read in more detail. I really don't want it to come to that. Honestly, I might just forget about it. Really not worth the trouble, but as I mentioned before I'm a bit uncomfortable with how GMG communicated with me, guaranteeing a certain credit to be issued then later saying, "Oops, we didn't mean the difference."
The digital game purchase heard throughout the world! *facepalm*
Well, at the end of my tale I'm glad to say GMG credited me the full amount of the error in the transaction. I told them I appreciated the help so far but felt that it was appropriate to fully adjust since I hadn't authorized the larger amount to be charged to my card. I mentioned that I didn't want to do a chargeback, but if GMG didn't fully amend the payment I may have had no choice. Apparently that was the straw that broke the camel's back. I didn't mean to do it in any threatening way and hope the CS rep didn't perceive it as such.
Anyway, happy ending for those of you who were following this intriguing tale.