Enterprise plan is very transparent and it's fee is calculated depending on number of users (one user - 200 USD / month (30 days)). Enterprise plan's fee is billed before the period starts.
Your credit card will be charged automatically for the next period before current period ends. You will find your invoice in Billing menu under Invoices. You will be informed if your credit card's expiration date is approaching. Failing to update credit card information will lead to your subscription being cancelled.