A few more notes:
* Do seat guarantee early if you can. If you wait until the last minute and cannot get through to MPC in time (due to engaged line, holding, etc.) at least 72 hours before ETD, you will not be able to do seat guarantee. It is also a courtesy to give more time to the airline to make arrangements on their side.
* Seat guarantee is not just for ticketing. If you change your flight while you are on the road, you can still have seat guarantee.