Google Check-out and promotions

I've started working on some enhancements to an ASP.NET web forms e-commerce project, one of the tasks is to add promotion codes to the check-out page.

Currently, the site is using Goo开发者_高级运维gle Checkout (via the GCheckoutButton that comes with the .NET sample pack), the trouble is I can't find any sensible way to add the promotion to the Google Check-out XML request...

Has anyone done this before, or should I just apply the discounts to the items before putting them into Google?

Thanks, Kieron

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It's not perfect, but adding the last shopping cart item with a minus value, and a sensible description did the trick.