My customers seem to have an ongoing problem with gift certificates and I think I've figured out why they are having confusion so often.
The usual complaint is that they've redeemed the gift certificate but it doesn't get applied to the order.
I tried duplicating the problem and here's what happens on my setup (1.39h) -
1. customer begins checkout and has a gift certificate code which they have not yet redeemed.
2. on the payment page, they enter the gift certificate code and a payment method.
3. Clicking continue brings the customer to the final checkout page with a message that indicates that they have redeemed the gift certificate successfully.
4. Gift certificate has in fact been redeemed but not applied to the order.
It seems that when a customer redeems a gift certificate *during checkout* they must also specify an amount to apply. I guess that makes sense but the "redeemed" message makes the customer think that they've done everything they need to do. And since they've now passed the payment page, there's no additional opportunity to use the gift certificate during the current purchase. Finally, the customer is puzzled and upset when they get to the confirmation page and there's no gift certificate amount applied to their order.
I've added all sorts of text on the payment page to try to make it clearer for customers, but this comes up several times a week, so that leads me to believe it's still confusing to customers.
One possible solution would be to automatically default to using enough gift certificate funds to pay for the whole order (or as much as is in the gift certificate balance) with an option to reduce that. But I don't know how to do that.
My Zen Cart defaults to using zero of the existing amount of the gift certificate until another amount is specified. I would imagine most customer want to use their gift certificate now rather than later so defaulting to zero seems counter-intuitive.