The Zendesk apps marketplace bills for your usage of an app for the month in advance. This amount is calculated from the number of agents you have at the time of billing.
So if you're using an app charged at $2.50 per agent/month and you have 10 agents, then your bill will be $25.
However, another thing that is calculated in your bill is what happened in the previous month. For example, if at the start of the last month you were only using 5 agents, and then precisely halfway through the month, you increased that amount to 10, then your bill will be adjusted to take that increase into account.
So, for the 5 agents who used the app for half a month, you will be charged 5 * (half of 2.50)
ie. 5 * 1.25 = 6.25
This amount will then be added to the $25 for the coming month, making your bill's total = $31.25.
If you'd like to discuss your invoice simply email firstname.lastname@example.org