The Checkout by Amazon team has released an updated version of the Zen Cart plug-in.
For: Version 1.3.9e

The updated plug-in supports the following features:

- Integrated with Instant Order Payment Notifications
- Dynamic Shipping and Tax Rates calculation using Callbacks.
- Amazon PayPhrase checkout integration tested
- Orders placed through Checkout by Amazon can be synched from Amazon through Merchant Feeds and managed within Zen Cart- Amazon order status history
- Reflects the statuses an order went through on Amazon and corresponding Zen Cart statuses.
- Has been tested to run with PHP4.4 and above.
- Checkout with Amazon button on the Shopping Cart page
- Amazon 1-Click and Express Checkout integration
- Signed / Unsigned Carts
- Ability to add custom data required by merchants via custom data modules

The plug-in can be found on the Zen Cart Free Addons page.