I would think the online ticketing works automatically. It wouldn't need a person to process it. You don't buy tickets online by emailing the box office. You buy them through the computer with your credit card.
The site says the box office is not staffed on Sundays except during performance. So if you get there at 6:30 or 7, I'm sure you'll be able to buy tickets. As I said, the show was not sold out on the night I went.