Conflict with Kite plugin

Hi!

We have recently released Kite Print and Dropshipping on Demand plugin, and the initial release had some issues for several users. One of them pointed out that there might be a conflict with this plugin and, after checking it, we found out that there really is an issue.

For some reason, when our plugin settings are being saved, a part of your code is called twice. Because this code contains require, the whole thing fails.

The fix is easy: please replace require with require_once in two lines in the function wpruby_init_royal_mail in royal-mail-woocommerce-shipping-calculator/wc-royal-mail.php, and the issue should be resolved.

Thank you.