Well ... actually I goofed ...

I was thinking of Gift Certificates not Discount Coupons ...

Discount Coupons are redeemed on the checkout_payment page ...

There isn't, at this time, a way to redeem Discount Coupons from another location ...

You can do a lookup for a Discount Coupon to see if it is valid and its restrictions ...

The biggest problem is that a Discount Coupon requires valid products in the shopping cart, before it can be redeemed ...

Then, once redeemed, if the content of the shopping cart changes and there are no longer valid products in the cart then the Discount Coupon is removed ...