I found this thread well after it was "live" and so I'm posting in case someone else is searching and looking for the same answers. First - the main answer that I'd been looking for was how little I could have remaining on my auto-pay to still get the autopay discount while paying off the rest using a gift card. I've been leaving $1.85 and things work fine. I suspect you can leave even less, but this amount fits for my scenario very well.
I will summarize the instructions that I got from other posts, just for convenience of anyone else reading this. I do this from my laptop/pc:
Login go to BILL -> Payment options
Chose "Pay another amount" - do NOT choose split playment
Enter your amount you want to pay from GC; click continue
If it asks you "do you want to make another payment" or some such, click "not now"
When it asks you how you want to pay -- select "add payment method" and then "gift card" and then "add method"
enter GC number and it will show you balance and process that payment.
Remember that you can then do it again if you need to split the payment across multiple gift cards. Just remember to leave some amount left to be paid at the end by your auto-payment method. Failure to leave the auto-payment amount will stop the auto-pay discount.
