When I get a request for a full manuscript, my normal routine is to send it boxed and priority mail with postage and label for return in the same box (not registered, not signature required).
I realize there's a problem for returns because of the Post Office's rules that packages over 1 pound must be hand-delivered. Last full ms. I got back surprised me by coming back by UPS. Would it mark me as a hopeless amateur if I asked if they preferred SASE by UPS or USPS when I get the request?
So you're the one.
Miss Snark has been looking for you.
This is a bad idea in so many ways that just thinking about it makes me emit noises only heard by dogs.
First, you can send your ms in a box if you want but it's more expensive. This is paper you're sending; it's not going to break. Put it in a sturdy envelope. Fold over the end to make a tight package. Apply shipping tape. Send.
For the return, using the same box is foolhardy. Chances are I ripped it to shreds opening it. And of course, I have to keep track of it while your ms is in the stack, passed around to be read, or generally NOT sitting in the box.
And of course I'm not going to the post office to stand in line to send it back to you.
Here's a novel idea: don't have the ms returned. Include a #10 SASE for my letter to you, and solve the problem.
Returning mss is a holdover from pre-copy machine days. If you have to laboriously type each page by hand, sure you want it back. Now it's less expensive to print a new copy then send it back.
Now, to quote Grandmother Snark when accosted by hardhat admirers on the streets of New York "wise up sonny".