Discover Debit Card?

No. This doesn't work for auto-pay based on my experience today. I tried a Discover debit card and American Express debit card for auto-pay but I for both cards, got an error message saying this payment was not accepted. The form fill recognized it as a debit card and even displayed an "Eligible for discount" message before I got a payment not accepted error. Had to use a Visa debit card. 

Almost all US debit cards should be accepted.  If it is an international debit card, there are a lot of variables that factor in to whether or not the system will accept it.