An add-on is an optional tool you can purchase to enhance and customize your referral program beyond the features included in your plan. They're available for an additional monthly cost and can help you supercharge your program. All Genius Referrals plans allow for the purchase of additional features. During your free trial, you can also purchase add-ons to try out. The cost of these features will be added to your invoice and charged to your account after the 30-day trial period.