I think it would be a nice cross-selling feature to include some type of variable to display similar products to what a customer might have purchased in a store in their email receipts.

For instance, if a customer purchased some labels that were of a certain size, the email confirmation, (as well as ship or update email templates) would have some type of way to display a product that that customer might also like; you see this on ebay, amazon, and other large eCommerce sites a lot. If a customer purchased item a, item b and/or c would be recommended because they are similar.

-Mike