The balance is prepaid: leads are debited as they are delivered. An insufficient balance pauses campaigns rather than creating an overdraft.
The NUMBER of top-ups is never limited: only the two amounts are. If you need more right away, support handles it on your file rather than making you chain transactions.
Two ceilings coexist, and the second one surprises people: a maximum per transaction, and a rolling thirty-day total. Both rise with your tier, which climbs on its own as you top up. At the first tier it is 500 € per transaction and 1,500 € over thirty days; at the last, 5,000 € and 100,000 €.