It looks to be working fine for me, just added 2 adults see below, lets me go to checkout with them, maybe its the child-adult combination you are doing. How many of each are you trying to buy?
OK now I am home I have logged into passholder section. Yes you can book with your AP's online at the discounted rate and have them delivered for $10 or willcall free.
I can book the 14th Sept at $38.95 no problems until I try getting them at AP rate of $35.95 then it won't let me book them. No problem booking other dates with AP.
If i book them over the phone do i have to have the AP number to hand? Only i dont ahve the cash to buy the 2 AP's for another couple of weeks but wanted to book the MNSSHP tickets for the 14th?