For Google Checkout, please see our integration procedure here (this also explains how to find your Merchant ID and Key):
If you want to offer buyers a choice of checkout methods without each product on your site having a different button for every payment method, then you would want to use our Cart buttons; just make sure every page that has any of our Add to Cart buttons also has at least one block of View Cart code as well -- you don't need a View Cart with every single Add to Cart, just one View Cart per page is fine.
Buy Now buttons would bypass the cart and take the buyer directly to checkout using one particular payment processor, so if you wanted to offer a choice of payment options with these, each product would need a separate button for every checkout method you want to offer, which sounds like what you wanted to avoid.
You can set up the CD+ebook as a separate product from the download-only ebook, and likewise with the DVDs. You can use our SwiftCD integration to have them automatically duplicate your CD/DVD and mail it to the buyer on-demand whenever an order comes in:
We have no means to support Amazon Payments. For sales coming from eBay or from some cart system other than E-junkie's purchase buttons, we can only fulfill digital product delivery for payments made by PayPal; please see our eBay integration page here: