I recently booked a flight with JustFly for travel on United and Avianca. I was very surprised to find:

-the charge on my CC was directly from Avianca, ticket was 134 stock
-the eTicket receipt view on united.com showed a different (higher) price than the one I paid, with a note at the bottom that an add collect for the difference was charged to some other CC (not mine).
-last 4 of my CC showed on reservation (typically on OTA reservations payment info shows as CASH or the like)

So it seems like their business model is buy the fare directly from the airline and charge you a little less than retail and pay the difference out of their own pocket? Then how the heck do they make money?