I'm sorry but I've never worked at place that had an honor system for snacks when people are working 60 to 80 hours a week. I work at CPA firm and during tax season our kitchen is fullly stocked. Nobody ever goes hungry that is the last thing we want. We have oatmeal, granola bars, pop tarts, cereal, top ramen, cans of soup, tuna packets with the crackers, mayo; instant meals like the black beans in rice (they are sold near the soup section), candy, cookies, tea, coffee, hot chocolate, etc... You name it we got it. I'm really surprise your firm would cut out such a cheap investment to keep employees happy. So, either they have already done this and discovered the employees abuse the system or they just don't care.
I think, you have done more than your fair share. I'd send out an e-mail to see if people pay up and if they don't then, stop or only buy with what money you made. Good luck!