If you book with the REACH NOW app, the invoice will be sent to your e-mail address immediately after your booking. The corresponding invoice is attached to this email as a PDF document. Please also check whether the email may have ended up in your spam folder.
In the latest version of the app, you can download your invoices directly from the app under Account> Invoice. If this option is not available for you, make sure you have your app updated to the latest version.