Envelopes,
Labels, & The Ease Of Ordering
by Herschell Gordon Lewis
The big advantage of a window envelope isn't that it's a better sales generator, but in that it can save you a lot of money. Since the label shows through the window you're using only one addressing label, not two. But why do you want the label to show through the window? For that matter why have the label on the order card at all, when you can just as easily ask the recipient to fill in the name and the address? You want to make it as easy as possible for your customer to respond, and by not having to fill anything out it makes it easier for them to order.
(Taken from Direct Marketing Strategies and Tactics)