You see the higher price if you try and charge a non-Tesla EV in the app.I think the "membership" is tied to the car when you own one but if you need to charge other EVs expect you'll need to sign up for a separate membership. The billing is obviously automatic via the car for a Tesla but for others it's not.
It'd be a nice perk of ownership to get a discount on a second car but we can all see why they don't do that.