I am in the process of converting opencart into a dual cart amazon / normal cart, so far I have done..
I have modified a theme to work with changes to the core files and database. and template This new site detects if you enable Amazon in the admin, if you do it disables a lot of the standard cart features from the admin and client side of things.
For every product you have an amazon enable switch (yes / no) you also have the ASIN. The end result is now a cart that can work with both amazon and non amazon products, take a sneak peek before I go live. http://www.crazypetfood.com
You will see a more information button that takes you to the product view, then a buy from amazon button for the enables products. At the moment only frozen meats are buy now / add to cart (normal cart)
There are still a few bits to sort out, I thought I would keep you posted. I would love to one day add this to vqmod, but so far it is problematic with regard to the DB changes.
More soon
