Go to “My Profile” in the side menu of the app. Then open “My Trips” and select the trip you need an invoice for. The invoice will be shown there, and you can send it by email or another way you prefer. You can find the same information by logging into your account on the website (here).
Usually, at the end of each month, an invoice for all trips is generated automatically in your account in the app (see “My Profile” -> “My Invoices”) or on the website.