Unofficial Skyrim Special Edition Patch

That's a world interaction event, not to be confused with a wilderness encounter. I did get the letter in my game with USSEP installed recently. Unless something's changed, world interaction events go through a cycle until all of them in the list (node stack) have been completed. This means that you're very unlikely to experience a specific event more than once during a playthrough. For example, if you've already received a letter of gratitude for murdering someone's enemy, you probably event. To complicate things, it seems to take very specific conditions for certain ones to occur. I've played a couple hundred hours in my current game and have experienced half of them at best. It's likely that the game's story manager is stuck (on hold) because one or more conditions have not been met on whichever one is scheduled to run next.

Correction for one of the sentences above that was incomplete:

For example, if you've already received a letter of gratitude for murdering someone's enemy, you probably won't ever get another for the same reason again.