This was an issue back in 1.1.7 I think, but it was fixed previously. Seems it might be back again there is one payment - authorize.net installed, it has been working flawlessly for many months.
In applying a credit to an account thatreduced the order to zero I am getting
Error: payment method required.
You can see by the screen shots that system has identified that there is a payment account noted but the previous fix to ignore zero balances seems to be broken.
this is a 1.2.6 Abantecart Version
thanks