I agree, that stuff is poisonous
That's why the pre-mixed stuff is better, you don't have to handle it so much
If you do it properly, it is possible to safely mix it and stuff, but if you're just doing it one time, there's a learning curve,…
The 16 ounce bottle says it treats "one complete outfit"
I just treat my boots and gaiters, once per year right about now at the beginning of the tick season. I've done it several times and my 16 ounce bottle is still mostly full. It will take quite a few years to use up.
I don't know what the shelf life is, but normally, after a few years it will lose effectiveness.
For me, there is no savings in buying a bigger bottle.
If you treat your entire outfit, several times per year, then the bigger bottle would be cheaper.
But, do you really want all that exposure to nasty poisons? They say it's safe if you let it dry, but you probably get some exposure, and you probably get some exposure when you apply it.
If you're in a Lyme disease or Malaria area then the poison would probably be a smaller risk.