If you want to include a custom message in the Successful Payment email notification receipt that is unique to each payment form, we provide a simple way for you to type in the message as you're creating or editing a payment form.
If you want to include a custom message that is unique to each payment form in any of the other email notifications, you'll need to use Liquid Templating and do the following:
1) Follow the instructions to customize the email notification that are sent to payers. (The same instructions can be used for the email notifications that are sent to you.)
2) In the Template Editor, follow the format of this Liquid Templating example.