I heard that Chip Mixer doesn't charge you a fee, just whatever you want to donate. I would recommend everyone not to donate anything only if they are really broke. Firstly out of honesty, and secondly because otherwise, if everyone did the same, the business would eventually have to close down.
It's also worth pointing out that donating increases your privacy. If you deposit an exact amount and then withdraw the same exact amount, then you have lowered your anonymity set. If you deposit an amount, donate some of it, and then withdraw a different amount, you have a much larger anonymity set.
Honestly if you use a non custodial wallets and don't have Bitcoin from illegal sources or you don't want to avoid tax ie., you don't have to seriously use mixer service.
Implying that the only people who use mixers are doing something illegal, which isn't true.
We've know for years that a very small minority of coins which are mixed are linked to illegal activity, and by far the largest source of coins entering mixers are directly from centralized exchanges - i.e. people simply looking to regain some privacy from the ever increasing surveillance conducting by centralized exchanges and their blockchain analysis partners.