Discounting google transactions without using coupon module

[Ashish]

I want to discount (1%) of basket total if customer buys using google checkout.
(As promotion to use google checkout)

My site isn't using any coupons and not planning in near future.

Is it possible, i can discount 1% if customer clicks on google checkout.
I created coupon on google side, but on checkout screen, i am not seeing any coupon entry box..
or even, can i hardcode somewhere only for google-checkout button?