Last time I used cash in an iron ranger at a city park (with envelopes that were provided to list details of who paid), the next day, I ended up with a note on my truck stating I need to pay up or face trespass charges, so it was a lose/lose. I've always used checks ever since, no issues.
Maybe instead of checks, perhaps the government should consider making a universal app/website... log on, purchase a pass via credit card, get a code number or code word, scribble it on a form and drop that in the iron ranger with one's license plate no. That way, the only thing that goes inside the box is just physical proof that someone paid; the money is safely (well, safety is a relative term when it comes to banks) stored in an account somewhere. Plus, there are multiple places showing proof of payment.