That's a contradiction in the show. In some places, they laugh about the concept of "money", and in others they clearly have and use money like you point out,

That's a contradiction in the show. In some places, they laugh about the concept of "money", and in others they clearly have and use money like you point out, and talk about "Federation credits". It's one of the really annoying things about Star Trek, because they clearly didn't think it through. For the episodes where they aren't dealing with extra-Federation societies like the Ferengi, it makes perfect sense to not talk about money on the ship. It's not much different than being in the Navy today: if you work on a US Navy ship, you don't have to pay to see the doctor, or to go to the mess hall and eat, etc. All that stuff is free. It's part of being a crewmember on a ship. It's the same on the Enterprise-D (or even Kirk's Enterprise), except they have even better benefits (e.g. replicators; on modern Navy ships there's I believe a little store where you can buy extra goodies they don't give away in the mess hall, but with replicators you wouldn't need this). Gene's idea of modeling Starfleet on the Navy worked out pretty well this way for a while, until they started dealing with jurisdictions outside of Federation control and then they had to retcon stuff like "gold-pressed latinum".