It doesn't actually charge you anything. You just have to put a card down to be considered a priority because now you potentially can spend money & therefore are more important then the other free-tier losers. /s It's still free tier & still free.

The free tier is also based on capcity usage, and not instances. If you want 3 cores on 1 machine & 1 on another, they're cool with that. I personally run Pangolin on a 1 core & self-hosted github runners on a 3 core.