Hola Anton, 

The lesson states that the postman comes “at 3 in the afternoon every day, and he is very punctual.” The way to express a specific time of the day is definitely with “de” la tarde / de la mañana e.g. “See you at 10 o’clock tomorrow morning = Te veo a las diez de la mañana.” 

You use “por” when you are talking about a part of the day. If you think of it as meaning “during” you won’t go far wrong.  So I might say “Hay mucho ruido por la noche” meaning “there was a lot of noise during the night.” 

Note: The lesson you are referring to [Using en/a/por in Spanish to express parts of the day] comes from the Latin American course. I don’t think it is common to use “en or a” in European Spanish to describe a part of the day. I am only familiar with using por in this context. 

I hope that helps. Saludos. John