allow more agents and charge by online agents only

Instead of charging per agent account set up, you should cap the number of users which can be logged in at the same time. This is better for teams where staff work shifts. For example if we have 2 weekday staff on the live chat with agent accounts and then another person who does the live chat on the weekends we would need either 3 accounts or sharing accounts (a little tedious). It seems like a better idea to control the maximum agents you can have connected at any one time and allow unlimited number of agents to be created. If you have to many people logged in just show an error / upgrade prompt.