I think you’re misunderstanding what I’m saying. The Federation is post-scarcity. They don’t need to worry about how much things cost inside their economic bubble.
Just because Federation citizens don’t need to think about money and buying things as part of their daily lives, that doesn’t mean things have no price. For federation citizens, that price means nothing and they’d likely never even see it. For a Ferengi buying something like replacement parts at a Federation station though, that price matters.
When you’re dealing with other cultures that do work with a monetary system, they have a monetary value for things. If you’re going to interact and buy/sell items back and forth you need to have some sort of system to facilitate that. So the Federation has a standard credit, and items are worth various amounts so the cost can be directly exchanged like we do currently comparing different currencies. That credit just doesn’t really matter to Federation citizens that don’t interact outside the Federation, which is likely the nearly all of them given the size of the Federation.
We only see it referenced several times because we’re watching Starships that interact with all these different cultures daily. We aren’t watching Mr. Wilson who lives on Earth and has never left Federation space. While he does travel, he goes to Risa for vacation, and that is inside the Federation still. So his daily work, recreation, and even vacation doesn’t involve needing any money.