Payment options such as Stripe (my favorite), Squarespace, and PayPal are pretty easy to incorporate.
There are a lot of software licensing managers out there. For those that work as a website plugin, when payment is completed on your website, activation codes are automatically generated for the relevant products and emailed to the customer.
Next, you'll need to make a connection between the user's plugin and your license database. Your plugin should send an HTTP POST activation request to your website. I don't think HTTP functions are part of the SDK. As I said earlier, using a portal script to communicate with your license manager means your plugins won't be hard-coded for (married to) only one license manager. 😉
Next, your plugin needs to respond to the returned POST to switch from demo mode to licensed mode.
You can, of course, store settings in Illustrator's preferences, but if the user deletes the preferences or upgrades to a new version of Illustrator, the info won't be there anymore. I recommend storing license status in an encrypted license file, and storing your plugin's preferences in your own preferences file.
Encryption, HTTP POST and GET, registration, and JSON-formatted preferences are all built into CORE which made it easy for me. If you incorporate them, I'd like to hear how you did it.