I inquired about this a bit ago, but I know you're quite busy and this is probably a low priority for you and your team.
However, I would like to know why the check out process for the PC Downloads is different than regular Amazon.com shopping in that you cannot put the purchase on just a card if you have any Amazon gift card/promotional balance. You're forced to use your gift card or promotional balance. I know this goes against most people's logic, but I would still like the option. Thanks for you support of this community!
Sorry I didn't get back to you on this. I'm not sure I follow you on your question. Are you saying you have a gift card balance worth enough to cover the cost of the game and don't want to put in a CC?
You should be able to input a GC code right under the "Complete Purchase" button once you click "Buy and Download.
Let me know if this answers your question, sorry for the confusion.