I have been going to the post office to mail paper tax returns "certified with return receipt" because we all know the gov "looses" things in the mail all the time...so tracking is KEY. But with covid going to the post office has become even more of a hassle.
What is the easiest way to mail things to IRS and FTB with "proof" of mailing/delivery?
No, I don't know that "the gov" (not sure who you mean by that) "loses" things in the mail all the time. If you mean the Postal Service, a return receipt isn't going to keep it from getting lost. That just means there is twice the chance of an item going astray (the original mail, and the postcard back). If you mean IRS, what does the return receipt prove? All I know is that it doesn't show what it was you mailed. You could mail an empty envelope every week or so, and save the return receipts in case you needed one next year.
When I need to show I mailed something, I use stamps.com to print a label with a tracking number. It's not foolproof, so I don't wait until the last minute to do it. If the tracking doesn't show it delivered, I mail it again. The best practice is to put the tracking number on the contents, before sealing the envelope. A First Class, 9x12 envelope "package" weighing half a pound may cost $3 to $4. I pay for a monthly subscription to use the service, but I think you can do it directly from USPS.
Clients mail me things by Certified Mail all the time. They don't know that it doesn't get any more special handling beyond what First Class has already. But it makes them feel better. One client just sent me something requiring a signature. My office assistant is away this week, and I'm usually still working from home when the letter carrier arrives. Eventually, someone will be there to accept delivery, although I'm not sure about handing stuff back and forth.
If you want to pay another $20 or so, use Registered Mail. It's overkill, but OCD customers love it.
IRS response when you ask about the 2016 return: "Yes, we received that envelope. No, there was not a 2016 return inside -- just 2017 and 2018."
I would mail each year in a separate envelope, and use tracking (or certified, if you must) for 2016. When more than one year is sent in the same package, they tend to be stapled together and only might be processed.