MSPs have a unified billing system across all their customer accounts. The MSP Admin can subscribe, upgrade, purchase additional add-ons and alerts, which can then be used across customer accounts. Since an MSP account admin has complete control over all the customer accounts, the subscription and billing are wholly handled by the MSP Admin centrally from a single console. Once payment is completed, an invoice can be generated for the same.

MSP's can subscribe to an MSP Plan or MSP Enterprise Plan to manage customers' service remotely. Details about MSP pack pricing and services offered can be viewed here. 

Site24x7 does not charge your customers separately. You can bill your end customer as per your standard process.