The Charging Plans post debit transactions against a site owner's account on the server (and an individual site owner may have several sites posting debits to the same account). These serve to reduce the credit balance in the account.
Each account has a credit limit, which can be adjusted individually if needed. As the balance reduces approaching this credit limit, the system estimates how long it will be before it reaches this, and you can configure warning emails to be sent to the site owner alerting them to this, typically seven days before it anticipates the limit is reached.
The account can be credited by the site owner themselves, using the system's integration with online credit card payment processing systems like WorldPay, PayPal and Authorize. The site owner is presented with a suggested top-up amount to keep them within their credit limit for a further 30 days, and they can choose to add that amount, or pick another amount to 'purchase'. When their transaction is complete they are shown an invoice which they can print out for their accounts.