How do I add a safe and real-time post-payment feature in my website?

If I want to implement pay-as-you-go on my website and payments have to be done for the minutes used by the user, not for days, then what is the most safest way to charge users?

Note that I do not want to hold payments again and again because the transactions might be too many and I don't want to flood my user with a lot of invoices. I can't ask for pre-payment as the user doesn't know how much time will he spend on the website.

The only way to ask user is to show final amount in the end of the session.

How can I minimize pre-authorizations (hold payments) on the user's account? Is there any way I can predict the total amount that'll be used by the user?