Add keywords for cost per item in the after purchase script field

[multinet-pmerk]

Google Analytics can show revenue per item but as the plugin only shows the total revenue the revenue per item in GA is always 0.

[itssimple]

This is probably better handled as a plugin to the plugin, so that we don't screw around with the current script support, as the one implementing the script should be aware that it is only the total sum that you can get currently.

[itssimple]

Making this as a plugin to the plugin, so all info will be handled in https://github.com/MultinetInteractive/eduadmin-google