I just returned home and saw my prudential letter on my table (I wonder why they have to send so many). Besides that, I noticed that on the envelope, there were some stamps depicting common street trees which I found interesting.
A check at SingPost website showed that it had collaborated with NParks to let the public know more about our 10 most common street trees in Singapore. More information in the pdf here. For those who want the whole collection, it costs only S$2.60!