Is there really a point to sending enough postage to have a novel-length manuscript returned? Doesn't it make more sense to just send the SASE for your reply?
No. There is really no point in sending a large envelope with postage for the return of your manuscript. By the time it goes through the mail twice, plus gets read through several times, it's really beat up. You won't be able to send that same copy to another publisher and most of the time, there will not be notes of any value in the margins.
If I do have notes that I think would be helpful to the author, I e-mail them and tell them that they have a week to send me a large SASE if they want the mss back. Then I date a stickie, slap it on the mss and put it on my assistant's desk. If the SASE doesn't show up by that date, I assume she tosses it.
A #10 SASE is all you need to include.