Optional features (such as Pro-Option, USB stick, external HDD, IP addresses and so on) will be charged on a due date. Therefore the amounts are generally settled on a monthly basis and not on a daily basis.
Due date is always the 1st of the respective month shortly after midnight. If optional functions are active for a contract at this time, these will be settled, regardless if and when the contract itself expires in the billing month.
Conversely, optional functions that are activated within one month (after the key date / settlement date) will be only invoiced on the next due date of the following month.