Can I buy an add-on to the program and use it later?

Purchase an add-on to your program at a time that suits you and use it whenever you like. After purchase, the add-on requires activation. You decide when to do this.  
Activate the add-on in the app on your account. To activate the add-on to your program, tap: More > Add-ons > Available, then tap the add-on for the selected period and follow the instructions.