So you want to add this message to the payment page during checkout?

If so, there's some text on that page next to the PayPal option which says "Save time. Check out securely. Pay without sharing your financial information." You could add to or replace this text with your desired message. To do this, you would need to go to..

includes/languages/english/modules/payment/paypal.php

Line 20 I think.

Remember to overide the file.