We are looking for a way to include Amazon Payments to a Magento CE.
How would we best include them to an existing store?
We've integrated it a number of times - Amazon are actually surprisingly pro-active (and eager!) about their support during installation; and will basically hand-hold you throughout the process anyway.
Contact your account manager - and they'll put you in touch with an integration agent.
I'll quote from Amazon's own instructions.
Install the latest version of the module (http://www.magentocommerce.com/magento-connect/amazon-checkout-extension-9731.html ) on your staging server (it can be deployed on the live server too, but CBA button has to be hidden there) and configure it appropriately.
As soon as you are done, Amazon will have a look at your integration in the sandbox mode and will switch the sandbox mode off (accounts need to be fully verified before they can test in production).
You will put a couple of “live” orders in the production mode and confirm, that orders are correctly imported into your backend and that every action in your backend (shipment confirmation, cancelation, refund) is correctly reflected in CBA SellerCentral.
Finally, Amazon again will have a final look at these sites and sign off your launch.
Amazon has a newer Payments service called Login and Pay. Extension for Magento is available here for US sites:
The extension is also available as an open source project here: