Did you note the stamp used.
My eyesight is not the best, but it appears to be the Liberty Bell and there are odd terrorist type incidents and threats that are in that area.
I was once a stamp collector and choose the stamp to fit in with the intent of my envelope.
Many years ago, it was common to do so, that is why we have Christmas, Love, etc style stamps.
Interesting that these show all ‘they’ fight against:
It looks to me like he just used the "Forever stamp". Everyone bought them when the price went up. I did. As I recall, there was only a choice of two stamps at that time. I'm still using the single sheet I bought then. I don't even remember what the other one looked like.
I can't see any significance in the stamp he used. I think it's just the most common stamp used by everyone.