Hi,
the easiest way is to replace the 'add to cart' button with a 'buy on amazon' button. But from the ecommerce point of view, you might lose customers because they can't buy an affiliate product and your own product in 'one-click'. So will they come back to buy your product?
I don't if Amazon allows you to 'forward orders' using their API's. That would allow you to transparently mix both products in your cart.
HJ