Right now I have 3 types of subscriptions at my site:
Also, I have a few add-on's that user can add to their subscription, e.g.:
Subscription: plus
Total = 15$ + 10$ + 10$ = 35$.
Users should be able to add/remove features during their billing period or switch to another plan.
How should I manage those features using Stripe?
Stripe doesn't natively support add-ons. There are a number of third party tools that add this feature on top of Stripe, which can be useful if you're dealing with complicated up and downgrades with prorations.
Using just Stripe, standard practice would be to create a subscription with the plan (Starter, Plus, Premium), and then listen for the invoice.created webhook each month to add the necessary add-ons.