If the bras are in good shape, check into local women's shelters or thrift stores. We also have a rescue mission that you can dontate things to and they'll give you a receipt so you can claim them as charitable contributions on your taxes.
Underthings, to me, seem like a strange thing to have if they belonged to someone else, but let me tell you....
My sister has an annual yard sale and her bras and nightgowns and slips are the first things to go. The first year I helped her, I couldn't believe she put that stuff out, but they went like hotcakes. I never would have believed it if I hadn't seen it. My sister is large busted and she doesn't buy cheap bras.
Women were happy to get things in that size for a dollar each.
Many churches also have "clothing closets" for families in need.
Surely, you should be able to find somewhere to donate them.
Best wishes.