Every paid call is reserved before it is sent and settled at its real cost. Each project has one monthly budget for everything it spends, US$ 50 for a new project, in Settings > Monitoring > Budget. Collection is also capped per month and per day (SELF_HOST_MONTHLY_LIMIT and GLOBAL_DAILY_RUN_LIMIT), and Admin > Usage and costs shows where the money goes and can set a brake for the whole installation. When a limit stops work, the Overview says which one.
Spending limits
Every paid call is reserved before it is sent: each project’s monthly budget, the collection caps and the brake for the whole installation.