Hello,
Please re-download the Mondial Relay plugin to get the warning fix.
A PHP warning is not a blocking error and do not impact the way that the plugin (the code) is working. It indicates an improvement for the developper.
In a production environnement, you must not display warning which would be considered as a serious security issue.
That kind of display should just be done in a test website.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.