They know they've got someone who cares, because you did it for free. Good going.

Years ago I worked at a neighborhood outreach (volunteer) one day a week. Same thing happened. The umbrella organization (Catholic Charities) offered me a paying job. I wasn't in a position to take it, but I think volunteering is often a path to paid employment. After all, they get to check you out before they ever pay you a penny.

I'm betting this turns into permanent employment, if you want it. Good luck!