Onpipeline is a CRM focused on Sales. It does not include administrative native tools for invoicing an billing. You should use your own billing system, likely connected to your accounting department.
On our side we can help you make billing information available within Onpipeline thanks to custom fields (i.e. payment status, etc.), and if you or your developers can integrate external data you can easily use Action Menus, Widgets and/or sync data with our API.