We went through this a couple of years ago: one kid had a tonne of lice in his hair, my wife got infested and ended up shaving her head. The other kid and I lucked out.
Regardless of what happens, we've got some short summer haircuts in our future.
I had pubic lice 30-years ago, and even then we didn't have to shave.
If you read the bottle, the "medicated" shampoo is nothing but bug killer (pyrethrin) mixed with shampoo. There is literally nothing else in it. Mix up some Raid and Pantene, good to go.
And before anyone chastises me for giving this advice or being stupid, pyrethrins are harmless to humans. They only fuck up insect nervous systems, can't interact with mammals.
Hedrin doesn't use pesticides. It's made of dimeticone and is very effective from my experience.
"PDMS is effective for treating lice in humans. This is thought to be due not to suffocation (or poisoning), but to its blocking water excretion, which causes insects to die from physiological stress"