Hello,
Sorry for so many questions. So when a User registers it seems like he can create lists, send emails etc without any subscription. Is it possible they are forced to sign up and pay for the subscription before using the service?
The idea is that you can set the customer to be added automatically in a group once he signup(see Backend->Settings->Customers->Registration->Default group). And that group can be as restricted as you may want from the customer group settings. Defining then Price plans and allowing the customers to buy them, it will assign the customers to the price plans attached groups (https://kb.mailwizz.com/articles/monetization/).