You can pay on the day, but it costs you more for the same ticket than if you bought it the day before.
The club have installed a new ticket system over the close season and there seems to be a few teething problems making getting a ticket a very slow process. I'm sure these issues will be ironed out over the next few matches. The problems were obvious at last nights game and they upsetting for the people selling the tickets as well as fans,
If you are coming to the next couple of matches and know in advance I would go for the print at home option if you can. If you are going to pay cash on the day ,go for your ticket and buy it as soon as you arrive and allow a bit of extra time, that's my plan anyway