Charities are sitting ducks when it comes to receiving mail! Don't assume it is an "inside job" either! There are many weaknesses in the system. Let me give you my story.
My Donor Advised Fund check to my church never made it to the church. It turns out, some other donation checks didn't make it either. After some investigation on the matter, it turns out that thieves were stealing from the church's mailbox on Saturday.
This is not a USPS problem. This is not an "inside job" church problem. This is a thievery problem. Blame the thieves.
The church's mailbox is in plain site of the administrator, so they thought they were OK on this problem. They had someone pick up mail on Saturday. But there was always a window with mail sitting there. Thieves lifted mail during that window. So the solution was that the USPS now holds all Saturday mail. The much maligned USPS is being very helpful here.
The same can happen with other charities, especially if their business address doesn't have a secure mailbox. Some don't or can't. Thieves apparently steal mail from charities because: 1) Some people send cash, 2) Checks can be washed.
We live in a sick society.