Translation:The post office is already closed.
The comma, or 読点 (とうてん), is used to mark pauses and breaks in sentences. It is often used after は in sentences such as this. Consider the English sentence "As for the post office, it is still closed." The comma in the English sentence is put in the same place as it is in the Japanese equivalent.
I thought ha is such a natural mark in the sentence, that it would not need a comma
"As for the post office, it is already closed." Wouldn't that be the best translation ?