Buy SandFly Pro
After paying you get a code to enter in SandFly, under Settings, SandFly Pro, Enter code.
A one-time payment gives one period, which starts when you enter the code. Nothing renews.
Automatic renewal is by card. The period starts when you pay, and renews at the start of each period until you cancel at Manage subscription. A reminder is sent by email 7 days before each payment, and also 30 days before each yearly payment.