Yes, it is. You will have 'a seat' on the trip you booked as soon as you get your confirmation voucher. The voucher you received shows your booking number and the seat number. Your seat number is guaranteed, unless the bus operator changes the seating in last minute to accommodate a lady/child/monk/handicapped person. If you were able to pick a seat during the booking process you will get it, unless something happened during your booking. In such a case the seat number is taken automatically by the system and you will receive the best seats available at that point. So the seats are guaranteed, the seat numbers are not!