Although you searched through Hipmunk, a flight reservation is always booked directly with the airline, online travel agency, or third-party provider. You’ll need to contact the site you booked your ticket with directly to make any changes or cancellations to your itinerary.
Not sure who you booked with? Don’t worry! Just check the statement of the card you used to make the payment. The company that charged you for the reservation is the company you booked with directly.