def METHOD_NAME(order_with_products):
    reservation = order_with_products.reservation
    reservation.state = Reservation.REQUESTED
    reservation.save()
    order_with_products.payment_url = 'https://foo.com'
    order_with_products.save()
    user = reservation.user
    user.preferred_language = 'fi'
    user.save()
    reservation.set_state(Reservation.WAITING_FOR_PAYMENT, None)
    assert len(mail.outbox) == 1
    check_received_mail_exists(
        'Reservation waiting for payment subject.',
        reservation.billing_email_address,
        (reservation.order.payment_url,),
    )