As long as your Postmates account is linked to an Uber account, your Postmates trip count will carry over and be visible on your Uber profile. Completed Uber deliveries will then add to that lifetime total.
The number of completed Postmates deliveries is also used to provide Postmates couriers with at least an Eats Pro Gold loyalty status.
Trip details for deliveries completed on Postmates will also be kept available through the Postmates app.