Frugally donating

I live in a large city, and see a lot of homeless people in my day-to-day. I wanted to put together bags of food/personal items. I'd probably make about 20 to start, but theoretically I would want to do this at least once a month, more if possible.

What are some frugal ways to do this? I was thinking of getting a Costco membership and buying bulk- would I be able to write off the cost of the membership for this? The other thing I was thinking was just seeing what was on sale/had good coupons for the week, and stocking up.

Does anyone have frugal suggestions of what I could put in the bags that would be the most useful? I was thinking a water bottle, a vitamin (I'll have to check FDA laws and whether that's legal), granola bars (cliff ideally if I can find cheap ones) and feminine products for women. I also knit, so once it starts getting colder in my area, I was going to knit some hats/scarves to put in bags.

All suggestions are appreciated/welcome!