The humor was pretty good, though the execution could have been done better, in my opinion.
One of the main issues I had with this is that while the letter is being read out loud, the reader could easily read ahead, and it could spoil/ruin some of the jokes; for example, when it says "I'm so sorry I slept with your dad," the viewer could read ahead, so I think it'd be better if that part is only visible after he said it.
The voice acting was very good, I liked it a lot.
I can see that you chose to use proper lip syncing in the beginning, though towards the end, there was this two-drawing mouth loop being used. I can definitely understand if you don't want to do proper lip syncing since it takes a lot of time, so if you choose not to do that, then my advice for you would be to make the mouth loops more complex.