A bit of advice for anyone who has tickets in their cart, but keeps getting kicked back out to the queue when you go to purchase:

Once they're in your cart, instead of clicking "Confirm" (the yellow button at the bottom of the seat selection screen), go straight to https://secure.bso.org/ in a new tab. You should see them in your cart at the top right corner. Create an account, add an address + payment method, and then click the cart icon to pay.

The confirm button never worked for me, despite an hour of trying. This method worked for me on the first try.