I found some very helpful solutions to safely editing the WooCommerce Endpoint pages here:
https://www.woothemes.com/2014/08/five-quick-woocommerce-customization-tips/
What I wanted to do is add some basic text to comply with PayPal’s Redirect Page URL requirements ( https://www.paypal.com/us/cgi-bin/webscr?cmd=p/pop/express_return_reqs-outside ) to my WooCommerce thankyou.php template, and I used the suggestion under the heading: 1. Custom Templates in the above article.
