If the balance is reached before the transfer has been finished, you will be informed accordingly in a ticket. The unsent messages will be kept in a queue for 7 days.
There is no further configuration of SPF needed.
The system calculates the payment for all unpaid messages once a day. The payment is rounded to two digits after the decimal point. As an example, if there are several messages sent with the numeric value of 0,003GBP, the system will round a price to 0,01GBP.