Why not ask for 50 cents or a new stamp in the envelope? Including a stamp is easier in an envelope, and it should make prepping a return envelope simpler. Require "forever" stamps to protect yourself from a postal price hike.
(And as general advice, if you're going to do a mail campaign like this in the future, it might be a good idea to actually chat to a postal carrier about it first.)
I actually know someone who has worked on those machines for decades. He never told me anything other than that the machines might reject your letter, or your thumb drive might fall out and they'd not be able to find where to send it.