You have many ways to make a purchase with a spy app:
1 - In their user control panel site where you can view your the data logs.
2 - Buy it from affiliate sites and then you will receive an email notification contains a license code (or called it as 'ticket'). Use this license code to apply it with your control panel site, the expiration date will be extended.
In case you make the purchase with an auto-renewal option as you see in the picture above, you can make payment first (because sometimes it let auto-renewal is ON as default). After that you can log in to your account where you make payment, follow the instructions here to turn OFF the auto-renewal option.