Yes it's legal to film inside a USPS building.
If you go into the post office lobby and look for a poster on the regulations it will state that filming inside a USPS facility is legal.
So if you go to film a USPS building inside or out and they tell you to leave then refer the postal workers to the poster on the wall.
I've seen some videos where people were filming the USPS inside and the USPS employees called the police and when the police arrived they told the USPS employees that filming inside or outside the USPS is legal and allowed.
You can even call the USPS phone number and speak to a postmaster and they will let you know that it's legal to film in the USPS and it's your right to do so.