When so means “so that” or “in order that” and implies the reason for doing something, it starts a dependent clause. That clause takes punctuation depending upon where it is in the sentence.
When it is at the end, it takes no punctuation.
…We called late in the day so we could tell her the news.
…We went through San Diego on our way to Yuma so we could have lunch with him.
…I went to the bank so I could cash my check.
When it is in the middle, it is surrounded by commas.
…She was walking, so she could go to work, toward her car that was parked on the street in front of the house.
…I decided to call, so I could give him the news firsthand, on my iPhone rather than wait until I got home.