OpenCV FR operates in Prepaid mode. Your first payment happens the moment you subscribe to a plan. Subsequent monthly payments happen later on the date of your initial subscription, or the last date of the month in case your initial date of subscription does not exist in the current month (for example, if you subscribed on the 31st of Jan, you next payments would be on 28th Feb, 31st Mar, 30th Apr, and so on).
For plans that include Liveness, your usage is billed in a postpaid fashion (for example, your April invoice will include Liveness API call charges for the March period).
your invoice and payment collection happen on the same date at same time, which is at the start-point of each billing cycle.