There are two flavors of monthly payments - $x every month, and $y for every post the creator marks as a paid thing, up to a maximum of $z - but you can't leave someone a tip for that one cool thing.

If a creator is on the $x every month scheme, then you could sign up for that, and then turn it off again. Patreon charges you immediately, so that you can't raid someone's paywalled content for free.