Extending a complentary month of subscription service

[likesmoney]

I run a subscription service.

Customers can subscribe for 3 months and 6 months.

I have two buttons and they choose either one.

Paypal then sets up an automatic renewal for 3 or 6 months, depending on which button they chose.

I would like to be able to extend a complementary month for referrals and need to know how to implement it.