Context: Article I, Section 8, Clause 7 of the United States Constitution, known as the Postal Clause or the Postal Power, empowers Congress "To establish Post Offices and post Roads".

Question: who has the power to create post offices?
Steam of consciousness below:
The relevant sentence in the passage is: Article I, Section 8, Clause 7 of the United States Constitution, known as the Postal Clause or the Postal Power, empowers Congress "To establish Post Offices and post Roads".
So, the answer is Congress.