Pete, I looked at the website. The voucher may be a ticket, tally, or points accumulated for a prize. I have used something similar with students with extreme behaviors. I had a sheet that I stamped for good behavior. A week's worth was worth a prize, a glowing note home, etc.
Students can pick.
The Reward has more meaning and effect if it is something they want. My kids especially like special jobs to do in the school or sitting by my desk at a table. Personally I think that positive rewards work wonders. So, you really have to get to know your students. I would keep it simple, however.