We generate monthly invoices for the previous calendar month on the 3rd of the following month. That invoice is due on the 10th. So we will send you an invoice for your usage in March on April 3rd, and you will owe that amount on April 10th. We give you a grace period to pay the bill until the 21st (April 21st in this example), at which time we may suspend your services (like your electric company).
We also calculate your costs at the end of every day, so you can pay for your hosting early if you like.