Promo and referral codes are applied to your next month's bill. When you upgrade to a paid account you will not be charged on the day of your subscription since we bill based on the number of contacts you have in your email list.  At the time of payment, we don't know how many contacts you may upload during the month. So you will receive your first month's bill at the beginning of month 2 and the promo code or referral discount will be applied to that bill.