Billing begins when your first SIM is activated or five days after your order is shipped, whichever comes first. You should expect to receive your bill within a few days, soon after which it will be due. Your payment method will be charged on the due date. In that first bill, you will be paying for your current month of service (the days since you signed up through the remaining days in a 30-day period).
The day that your bill is due will be the same each month. If you incur any additional usage charges (such as International calling or roaming), the charges will appear on your next month’s bill. The same applies to any late charges incurred.