Ideally, you would photocopy or photograph what you are sending together with the
envelope, attach a receipt from the post office showing mailing,
delivery or receipt as the case may be (different circumstances require proof of different things), and the initials or signature of the
person sending it.
You have to imagine that back in the day when we had nothing more than a typewriter, stamps and
envelopes, that
people sent in their «resumes» via
delivery pigeons and they dropped rolled resume packages at the doorstep of your prospective employer.