Hello,
I did not take a look at your plugin since it would be a bit far from our support.
Please understand that there is a line between "custom development" and support on the product (and you're using an account without licence attached).
If you want to make a subscription plugin, you should take a look at existing subscriptions plugins.
And you will see that the variable "$pluginConfig" is not used in theses plugins, since the configuration is displayed directly in the subscription edition page.
Regarding the fact that you want a setting for an API URL, you should use the Joomla configuration part, via the XML file for such thing.
HikaSerial plugins internal types change how the plugin is used by HikaSerial regarding the integration.
But for every types, you have access to all triggers. Just that you won't have access to the same "integrations" via the HikaSerialPlugin class.
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.