Why are you even shovelling? It's not your property, it's not your responsibility, and you're not getting paid for it. Right now, you're doing volunteer work for your LL and are even thinking about donating the supplies. Plus, what if you get hurt shovelling, do you think your LL will pay your medical bills and lost wages?
Sorry to be cynical, but this is NYC and your LL is taking advantage of you. It's not your problem, the LL should either do it themselves or hire someone to do it, not count on the naivity and kindness of residents.
I'd immediately stop removing the snow and write a certified letter to the LL informing them that you expect timely removal of snow from the sidewalk and stairs. They are required by law to remove snow and ice in a timely manner, and can face fines from the city and lawsuits from pedestrians, perhaps that's what will motivate your LL, but you shouldn't be the one removing the snow.