We tried that earlier too. That still has the problem where customers will forget to apply the coupon, and for the most unusual reason, google checkout won't let you enter in coupons at the stage in the process Zen Cart dumps the user into it at.
The whole Google Checkout explanation for this is bizzare, they blame how it updates things in the cart for not being able to use custom coupons in the cart, but mention that their premium service implements the cart differently.



