I have a custom pay-per-click advertising service on my website. Clients subscribe to the service free of charge, but are metered throughout the month with a simple clicks x rate formula. At the end of the monthly billing period, they will be charged for the amount of clicks they racked up.
My question is whether or not it's okay to just straight out charge them and send them a receipt, or if I need to send an invoice as a warning and wait a period of time before charging their card (I gather their card info upon signup). Keep in mind that the clients have a dashboard they can visit at any time and see how many clicks they have so far in the month, so the charge shouldn't be unexpected if they are keeping up with the dashboard.
If indeed I do need to invoice and wait a period of time to charge, how long would make sense?