Were you trying to make partial payments or pay in full? My experience is if auto pay is on, you can use a CC or GC. But that option is only available if you're paying the contract in full. Obviously this still won't work if your balance is more than $1K and you're trying to use gift cards.
In that instance, you'd have to turn off auto pay in order for the system to allow partial payments and use the multiple GCs to pay it off. That's why right now I'm getting all my contracts just under $1K, so later I can pay them off whenever without hassle.
