Documentation -> FAQ -> My Account -> Answer
How does Monthly service compare to Yearly service?
Monthly services automatically renew each month using the credit card on file. We only accept credit cards for monthly sevice renewals.

Yearly services do not automatically renew and you will receive emails when your service expiration is getting near. You can renew your yearly service with either a credit card or request an invoice. All yearly renewals receive a discount equivalent to two months of monthly service.
My Account FAQs