Just taking a quick look,
Post Offices are discussed here What makes Post Offices a "great place to be social"? and includes an old quote “ACCEPT. Connects and unites people around the world.“ I am certain there are more threads regards this.
In regards to post boxes we have it’s own thread UK Postboxes Topic . Some in the UK are eligible and are regularly accepted (apart from mine 
).
I would consider the reason that all post boxes / phone booth not solid accepts but Post Offices are is the same as Bus Stops Vs Bus Stations.