Openpay LIBRARY

Hi, while I can't guarantee this will solve your problem, in both cases above (with my limited knowledge of the subject) I can see no mention of configuring the autoload. So CI doesn't probably know where to look.
Someone please correct me if I'm wrong here but the Openpay library is not namespaced so you will have to namespace it yourself in order to use it in CI. Also, it seems there are files int here with multiple classes defined. Not sure if that's gona play nice with CI.

So, once you namespace it, in app/Config/Autoload.php you need to modify the __construct function. Where the $psr4 array is defined, add something like this to the array definition:
'Openpay' => APPPATH . 'ThirdParty/Openpay'

Thanks for your quick response Jan, I will start with your suggestions.