there is no way to use saved amount on AmazonPay to gift Twitch gift subscriptions or even to get yourself a Subscription.
I have an amount saved up on my AmazonPay and tried to use this before having to use my debit card/ credit card and there is no way for this to be taken from the amount saved on AmazonPay first. For example I wanted to gift a subscription to another person and would of rathered use my saved amount that was on AmazonPay which I loaded with my debit card. So scince I used money from my bank to load the AmazonPay why bother having to take from my real life bank account? Also would perfer to have to type in the debit card info just incase for some odd reason maybe out of the blue there was a keystroke snake attacking my computer and my antivirus program didn't catch it just an example. So why spend money if the money is already on your own account?