Shopping with gift certificates

Hi,
we are testing a Zen cart for a customer before we launch it live.
I am having some troubles when shopping with a Gift certificate....Sometime the cart is addind the amount of the GC to the total, sometimes it is taking it out, but either way the total amount due at the end is not the correct amount (sum of the item,shipping,tax-GC amount).
Any ideas why is this happening?

Thanks

You will have to supply a lot more detail here... You say it is SOMETIMES doing one thing, and at other times doing something else. I further infer that sometimes it is behaving correctly and at other times it isn't.

Well... if it is behaving correctly sometimes, then I would suggest that your errors are not software-related, but operator-input related.

Remember... Gift Certificates are not PRODUCTS in the same sense as your physical products.

GC's are just a way of giving a CREDIT BALANCE to a customer, and making that credit balance either partially or wholly transferable to other shoppers.

We need a detailed example of your CG structure for instances when things go RIGHT, and a detailed example for when things go WRONG.

But I am fairly certain that the error is operator-input - not software.