You should let people know not to send coins, too. Coins can get violently ejected from postal sorting machines and cause injuries.

(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.)

You're right that people should not send coins in regular envelopes. But of course people mail coins, keys, and binder clips pretty frequently. Do you think USPS employees stand around an open sorting machine just waiting to get hit in the face with a paperclip?

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.

Fun project, cheers!