The more you look at (forms similar to the one mentioned above) and do some mock orders, you'll see what's up. That's the only way I can word my post without it being denied.
Cheapest Cost: If your group is 2-4 passengers, order the VOOM package under just 1 passenger's name. Click on 2-4 devices.
Considerably Higher Cost: if your group is 2-4 passengers, order the package for everybody, click on 1 device. Might be preferred, but it's not required.
Insanely Highest Cost: if your group is 2-4 passengers, order the package for everybody, click on 2-4 devices. Big mistake, but very common.