You need to first create a new AffiniPay account or use an existing account and connect to it from Global Settings > ePayments. Then you need to define the online payment settings there. YOu can choose any one of the AffiniPay brands, depending on your industry: ClientPay (DesignPay)/LawPay/CPA Charge/AffiniPay for Associations. Check Core Help Center > Global Settings for details. After the setup, the ePayments option is enabled for all new invoices. You can enable this option for existing invoices from the Invoices screen. Check Core Help Center > Invoices for details.