iTelescope offers both session and billing rates for the telescopes.
https://www.itelescope.net/session-vs-exposure-pricing
It is up to the member to choose the billing model. iTelescope offer the benefits of both billing models.
Session billing is required when some special features of the telescope are used, as it can take additional time overheads for the telescope to perform these actions. Session hourly rates are typically at a lower cost than exposure-based billing.
Exposure billing gives you certainty on how much it will cost you to take your images. If you image for 1 hour, you will only be charged for 1 hour of imaging time. Exposure based rates are higher as they factor in the telescope overheads.