Is it during the initial sign up process or when activating the service?
I have tried enter the referral codes in the Promo Code box while signing up for the $10/5GB plan and I'm getting the Promocode not applicable! error.
Can I combine referral with cashback through Shopback?
It turn out the referral code works on the Promo code box for any plans above the $10/month.
Not all referral code works as the person might not be a current customer.