Those water flavor squirts, mio or crystal light type stuff. I’ll drink plain water over just about everything else (egg nog is the weakness and exception right now…), but the various lemonades or fruit flavors are always nice to have around. I wouldn’t be surprised if something in their composition is not good for you.
A slightly more titillating answer would be lube. You’re putting something on a mucous membrane, and it’s almost guaranteed that some will be absorbed or ingested.
If I remember correctly, they can be anonymous. If that’s the case, they wouldn’t really be easily taxable. Still, we are talking about the government here, and if they tax lottery winnings, I would bet they tax rewards.