There are two ways you can accomplish that, but first be aware that not all payment processors can auto-redirect buyers after checkout; most only provide a return/finish/continue link on their completed-checkout page, which the buyer must voluntarily click to proceed to any thank-you or redirect page.
First, we have a built-in method to cross-promote your products; on the last screen of each product's configs in your Seller Admin, you can select other products to promote on our standard thank-you page after that product is purchased.
Second, you can specify a Redirect URL for each product, or a common thank-you page URL for all your purchases (the latter overrides the former), so when the customer decides to proceed past checkout, we would redirect them to your URL instead of displaying our standard thank-you page.
