My Rabbi Meir pushka is a promise to pay when it gets full as I can only send the funds online.
Therefore, if I need some gelt, which usually only happens for a rare Friday afternoon emergency because my local grocery doesn’t take MasterCard and I risk an ATM line that can push me past lichtbenschen since erev Shabbos is peak shopping time for non-Jews as well, I pull cash out and write down what I took or if I’m really out of time I shove the change from my purchase and the receipt into the box so I know what I took. Normally I replace it with more that what I pulled out.
In the end, I usually round whatever is in the pushka way up anyway when I count it so it hardly matters.
My local pushka does go to the shul/shul chessed as cash. I don’t use it for myself except to make “Chelm change” where I’ll take a big bill and change it for far less in smaller ones because I need change and I’d rather give tzedoko than buy something I don’t need just to break a big bill.