For using Coupons...

Login to google checkout. You need a google checkout account even as a customer in order to completed checkout with google. As is logging in so Google know which credit card to charge :) Once logged in then there is field to enter coupon and the customer can enter coupon and click on apply. Amazingly it actually works!
Sorry but dont see it. I logged in and can see my order in the table. I can choose shipping methods from the drop down. I can see the Place Order button which places the order. No coupon option.

I've posted it on Google but no reply. Will keep pushing them.