I don't know what happens if they simply try to pay off more than what is on the card. I understand it probably prompts for the payment amount, but if you pay the full bill and the card does not have a large enough balance, what happens? Does it pay the full bill and clear the card? Does it get declined? or does it pay up to the amount on the card and then leave a remaining balance?
I assume it gets declined as I've never accidently short paid my bill, and if it would automatically adjust payment to the amount on the card, there wouldn't be a need to know the amount on the card up front. Although maybe if they would try to pay the full amount, they will be pleasantly suprised that they don't need to know the amount on the card.