How do payments work?
Payments are made monthly based on mutual trust. On the 20th of the month, you will get an email reminder to send payment for your next month of service. I'm trusting you to do the math. You would need to pay for the next month by the 27th (net-7) of the current month, else I would discontinue service at the end of the month. This is why I ask for your Ark address, to verify you want to continue service based on receiving the correct payment. If service is terminated, you will have to sign up again.
Price should be calculated from CoinMarketCap at time of payment. You can also use the calculator provided which pulls current Ark costs and does the math for you. Slight fluctuations are expected, but if I notice your payments are always short, I won't warn you- service will be terminated. Honesty is the best policy here.