Oh hey, so, I worked as a postal clerk for 3 years and then was the manager of a post office for 2 years following that. I also worked on the same street as a major jail in a large city, so I saw A LOT of mail for people in prison. Working for so long in a post office taught me A LOT about how in this country (Canada) we systemically and VERY PURPOSEFULLY prevent the impoverished and imprisoned from being "rehabilitated".
The vast majority of people sending letters to prisons are sending just photos, usually family photos of kids, dogs, partners, etc., or they are sending cheques for prisoners to buy commissary (we know this because the senders will usually put the photos or cheques into the mail while they're at the post office, I promise I have never opened someone's letter). Here's what frequently happens to people sending tracked items to prisons:
The customer who sent the letter comes in around a week to a week and a half after sending it. They are upset with the post office because they contacted the person in jail they know and they said they still haven't received their letter or commissary! I track the letter. The letter arrived 2 to 3 days after it was sent, and there is confirmation that the letter was received at the address it was sent to. I confirm with the customer they put the correct MAILING address of the prison (sometimes people put street addresses for businesses and institutions when it should be a mailing address). They usually have, because more often than not this is not their first time sending something to the person they know in prison. And because my girls and I know the prison address formats, we can usually spot if they're sending it to the wrong place and look up the correct address for them.
So I tell the letter sender the prison has received the letter. The customer says "I contacted the prison before I came here and they say they never got it...." I print the tracking and confirmation that the letter was delivered to the prison, with the date it was received on, for the customer so that they can show it to the prison because the prison is lying and withholding the mail from the prisoner.
This happened frequently at my job, and led to me implementing a policy of always recommending customers sending mail to a prison to get tracking for anything they could not simply print another copy of. It was certainly not an ideal situation, because tracking in Canada is exponentially more expensive than regular letter mail, but fighting with the prison to make them admit they actually HAD the mail that had come up so many times and was such an issue that once a customer had to do it once, they always chose tracked letters after that. And you have to keep in mind these are people who are usually strapped for cash themselves and sending as much as they can afford for commissary. It really sucked that they were also paying extra fees so that they could actually ensure they had PROOF of their claims if the prison tried to lie to them.
This isn't even getting into when my customers were people who had just gotten out of prison and were trying to set up their lives again, and job hunt. They would often come into the post office seeking to rent a P.O. Box, because they didn't have a fixed address. Then we would have to inform them that in order to get a P.O. Box you need an address and government ID from a strict list. Generally we would recommend they just open the post office box jointly with one of the people they were staying with or a friend or family member. Well, here's where we ran into the SECOND problem: you can't USE a P.O. Box as a residential address, which makes sense, as you obviously cannot LIVE inside your P.O. Box, but it brought directly to my attention the sheer amount of government programs or systems that required a fixed residential address for the participants, even just to receive THEIR OWN MAIL. There are types of government mail that cannot be forwarded (they will say directly on the outside of the mail DO NOT FORWARD). So as a person coming fresh out of prison, attempting to secure a safe, reliable way to get your mail, you can open a P.O. Box and still not receive all your mail, including things like ID CARDS (which many, many prisoners informed me they no longer have after exiting prison, of course). They won't allow you to put your P.O. Box as the mailing address, and it isn't possible to forward these mail items if they're marked against fowarding because it's against federal law.