While on the Moon map, you must buy almost all the doors.

Before you get to the Megadome there should be a room with about 5 or 6 desks in it.

Hold X on every desk and out of nowhere your person will take out a little box with a stick thing in it (warning: when you get this you can no longer hold a space suit).

By the way this works on everything including the prize box. It's called a refund and you can keep using the prize box (without payment) until you find something you like.

It also works on vents and erupter core (the thing that let's you breathe).