To reduce transaction costs and prevent a large monthly number of invoices we bill services with a low monthly cost (e.g. (Premium) DNS Hosting, Cloud Storage and Docker Containers) on the first of the month instead of the day of order.
The moment you place an order for any of these products we bill it up to the first day of the following month. So that means if you place an order in the middle of January you only pay for half a month. Starting in February you will get invoices on every first of the month.
We bill some services not monthly, but annually.