@zois Got it you create the data behind the invoice. OK different to what I am used to. I was envisioning an ERP type application or project management application that provided the data for an invoice.
@jakeinatx Sell the billing as a part of the service and inform your customer that this is needed. Give them a terms service page as part of your service. They would probably need a separate API key or whatever to do billing on their site anyway. Or don't give them a billing page. Have them do...