Out of curiosity, what region are you in? I live in a city of ~80,000 in the northeastish US and I’m not even sure it’s possible to be more than 5 or 10 minutes from a grocery store here.

That’s assuming the key and message are entirely independent. If you or the recipient isthe type of person or doing the types of things that would attract surveillance from a nation state (because realistically that’s the one of the only scenarios where non-esoteric privacy practices might not cut it), it’s not unrealistic that they’d intercept both your digital and physical mail and would be able to correlate them. At least with public key encryption, the private key is never actually in transit.

This is how all modern cryptography works. A deterministic cipher is functionally no different from pig Latin when it comes to actual security. An electronic solution like public key cryptography is infinitely more secure. If you’re especially paranoid you can generate the cryptotext locally and send it by email; that would be much safer than anything you could achieve by hand.