There is the OSCommerce add-on for the Amazon Checkout.
This ad-on is said to update the Admin UI with order details and accept dynamic shipping costs, unlike the Zen-Cart add-on for Amazon checkout.
Has anyone tried this OSCommerce Amazon checkout add-on with their Zen-Cart?
I think it should work, since OSCommerce and Zen-Cart has similar directory structure.
Please let me know if this OSCommerce add-on is working for you with Zen-Cart?