You will need to edit yourdomain.com/wp-content/plugins/paypal_payment_terminal/paypal.ipn.php lines 40 to 51 for admin email and lines 59 to 68 for customer email. you will see code like this, just edit the required text.

Also, please note, line 40 & 59 can be changed to your company name and email, so that emails would come from you, and not from your site default address:

Edit the line like this:

$headers .= “From: ‘My Company’ <[email protected]> \n”;