You can upgrade your plan from free version to paid version within the plugin itself. Head to the Settings section -> Billing to view/upgrade your plan.
As of now, Optinly supports payments via Stripe.
If your billing invoice exceeds/or you have failed to renew your plan, you’ll receive a notification asking you to do the same and you’ll be downgraded to the free version.
All your card details are stored and handled by Stripe and they are safe and secure.