In our area, the charities actually ask people to put random foodstuffs in a certain bag and leave it on the porch for volunteers to pick up later. If they did

In our area, the charities actually ask people to put random foodstuffs in a certain bag and leave it on the porch for volunteers to pick up later. If they didn't want that food and would prefer money, it'd be a lot easier to ask for that. That leads me to think that they do want the food, but also that you're probably correct in thinking that people aren't likely to donate at all if it's not food directly.