You can configure integration with one or more credit card merchant gateways. Prepaid customers can then top up their accounts on their own. Postpaid customers who have their payment type set to "Credit card" will have payment taken from their credit cards each time an invoice is produced (normally at the start of each month). If the payment was successful, the invoice is marked as paid. If unsuccessful, it is marked as payment refused.
PayPal integration, for prepaid customers only, is enabled by default - though you should still read the PayPal page below. All others need to be enabled as follows: