I think maybe there has been some confusion here, so I will try to explain how things work.
It is not possible to disable the Common Thank-you Email for a specific product -- by definition, it is sent for every sale.
If the buyer finishes checkout and proceeds to the thank-you page before the payment processor has completed actually processing the payment (withdrawing funds from the buyer's account and committing them for deposit to your balance), then the buyer would only see a generic thank-you page asking them to check their email for further instructions. Therefore, you DO want to send a thank-you email for digital products -- if not a Common Thank-you Email, then at least separate emails for each product.
After payment has completed, when the buyer proceeds to the thank-you page directly following checkout, or using the [%thankyou_link%] sent in any thank-you email, that will bring up a combined thank-you page for every item in their order. Product-specific thank-you page customizations (from the "Message (HTML)" field of each product's settings) will appear after each product itemized on that page, followed by common thank-you page customizations (from Admin > Account Prefs > Common Thank-you Page HTML). Here's an example page to give you an idea how this would look:
https://www.e-junkie.com/ecom/rp.php?t=jg-kqswfja3e6cfcfd&c=fup
If the buyer only purchased a Redirection product alone which has no product-specific Message (HTML) thank-you page customization, and if their payment is completed by the time they proceed to the thank-you page after checkout, their Redirection will be activated instantly, and no thank-you page would be displayed at all. Otherwise, if the product has thank-you page customizations, then a normal thank-you page would be shown with the customization and a link to click to activate the Redirect.
If the buyer reaches a thank-you page from the [%download_link%] included in a product-based thank-you email, that page will only show the product's name and link, then product-specific thank-you page customizations from that item's "Message (HTML)" field, then any common thank-you page customizations. Again, if this is a Redirection product without any product-based customization, then the [%download_link%] would activate the Redirection instantly.
