During checkout, I have discovered a circumstance when I apply a lesser Gift Certificate amount than the total purchase, checkout allows me to Confirm My Order when full payment has not been met. In other words, I am NOT prompted for a payment method when the Gift Certificate apply amount is close to the order total. This means an order can be completed without full payment.

Gift Certificate balance: $100
Order Total: 64.58
Apply Amount: $64.50
Confirm Order Total: $0.08

On the other hand, if the Gift Certificate Apply Amount is only $50, checkout DOES prompt for an alternate payment method.

What is the "leeway" on lacking full payment with Gift Certificates for order completion?

Using Zen Cart 1.39d