> think about someone riding around on buses with a negative account balance that never gets checked because the card scanner is only synced when the bus comes in to the depot

The system here checks directly.

But yeah I guess transaction fees, we typically don't see them but the merchants do. I know of only two places where it's visible: a mobile kebab store charges 10ct and a local ice cream parlor just doesn't have it because of costs. Other than that every place I know offers it without (mention of) fees.