When a booking or check-in is created, Optix will always first look to see if there is an available plan allowance to which it can be charged. If no allowance is available, an automatic charge will be raised for the applicable user or team. These charges can be invoiced immediately, or added to the users upcoming invoice.
From the Settings > Features > Invoicing > Invoice policy, select either 'On a new invoice, due today' or 'On the next pending invoice' depending on how you want to invoice these charges.
Need more info on Settings?