Add your PayPal account email address in the PayPal Account E-Mail Address field.
To add a PayPal button to the email body:
Tap Customize E-Mail
Tap on the location in the text body where the link will be added
Tap Add Field and select PayPal Link
Resulting email example:
The PayPal button will appear automatically on the PDF invoices if you add your PayPal account in the PayPal Account E-Mail Address field (see above).