Hi,
You don't really need separate return pages.
The customers which bought your service will not see the download links for the digital product on your thank you page. They will only see details about their transaction for the service product which you are selling.
This is because the following shortcode which you should be using in your thank you page is smart enough to display the appropriate information for the relevant product:
[wp_eStore_transaction_result:end]
See this page about how to correctly set this up.
http://www.tipsandtricks-hq.com/ecommerce/wp-estore-instant-digital-product-delivery-499