There is an allowance for unlicensed sales at religious or charitable events, and I also think it would be okay if you were simply giving your food away as the donation itself, but I don’t think this private nonprofit business would be included in those. This question really brings up a gray area that I don’t think has been covered in the food code, so it would be up to the health dept to decide and you should call them. But my guess is that this would not be allowed without some kind of license, but the fact that it is an optional donation might have an impact. I’d be interested to hear if this isn’t the case.