(1) Alice wants to send Bob a physical letter.
(2) Alice also wants Bob to be confident that she sent the letter and not an imposter.
(3) Alice is confident that a potential imposter will not be able to intercept any letter she sends.
(4) Alice has previously in-person provided Bob a RSA Public key.
Can Alice use a RSA Signature to confirm she sent the letter?
What is the best approach Alice can take, that makes such verification as easy as possible for Bob?