I definitely think that configuring your Plus subscription should be made easier, however I don't agree that a tick box like that is necessary. It already is stated that it is a monthly subscription and you can't really toggle auto-renewal elsewhere anyway. That tick box would essentially just be asking if you want to pay for a month, which isn't really what a subscription is.
Perhaps it could be done on the CubeCraft store website, dunno if that's even possible though, since there aren't really profiles on the website where you can configure your account.
Though if there was a sort of account system on the website, you could have access to a settings page where you can manage your payment method, auto-renewal could be an option and there could be a 'cancel subscription' button with conformation if you wanted to cancel it. It would also be useful to display when it will renew/end too.
This is just an idea of mine that I personally think would be more practical in the long run if possible. Though this isn't necessarily a burning issue as you can still contact support and they will help you.