Linda,
I think you are better off using older machines with a parallel port than new machines with USB. The USB solution requires you to install the Zebra drivers. We found them to be slow, unstable and unreliable. We had nothing but trouble with the Zebra printer drivers when we used Shelby for children's check-in (back in the day).
We considered using the USB solution once (because we forgot how bad the drivers were). Our 5-year old Compaqs with Celeron processors were faster at printing than a 1-year old Pentium 4 machine (at that time) attempting to print via USB. Nevermind that the driver could flake out on you while your trying to check in a child.
Clete Rodriguez
Director of Technology
Faith Community Church