I also need this functionality. Seems like a very basic discount code feature. I can't believe it's not part of the core Shopify product.
Has anyone figured out how to do this?
100% follow up with your stores MSM If you need this feature in checkoutscripts so the request isn't lost in the public forum.
@Copagusa a workaround is to just use the ajax cart-api in the checkout templates to add a product to the cart during a part of the checkout.