After you have set up your ePayments profile, you can specify the online payment options at the client, project, or invoice level. The new invoices of a project inherit the settings of the project, while the new projects of a client inherit the settings of the client. The payment link is then available in the detailed view of the invoice and the invoice PDF. It can also be embedded in the invoice emails you send out. Check the Core Help for details on setting the online payment options at the client, project, or invoice level.