I was able to purchase without issue 30 days in advance for the holiday travel period. Simply chose the 'with Oversized Baggage Area' under 'Seat Types' and was able to purchase without issue. You can see your chosen seats on the 'Seat Map' - oversized baggage is the last row of the selected car. Tickets are automatically issued in the app and can be added to Apple Wallet if you're using an iPhone.