We are carbon based life forms, our being alive is just a fancy way of taking carbon in a solid form (food) and turning it in to CO2 for plants. So you either ship food and O2 to the moon and CO2 back to Earth, or you build up a supply and start the cycle there.

Presumably if you are running some sort of hotel, you ship enough O2 and food to cover your guests travel and stay. Then only ship back what they need to stay alive on the ride home. Eventually you build up a large enough supply that you don't have to ship as much, reducing costs.