Honestly, you should charge like $1 for each operation. People will trust it more that way, as you're not incentivized to snoop on their files (or put 500 ads everywhere).
Also, how would charging work when you're offline? One of the features of the tooling is that stuff works offline, seems maybe charging for each use wouldn't quite match up with reality.
(I already ruled out ads, since I don't want to clutter the tool and also most ad networks are not exactly privacy friendly which one of the main ideas behind the tool)
I always wonder if casual gratitude adds up to as much as ad networks. Probably not.