jason, you should see somewhere yellowish metal containers of PCK (Polski Czerwony Krzyż - Polish Red Cross - there should be a phone number on it, if you want to contact them), they may be stand-alone or close to some recipients for selected garbage (paper, glass) and you may throw your clothes there. Of course the clothes should be washed and packed in plastic bags.
but if you want to sell them for a penny, a 2nd hand store is usually called in Polish "sklep z używaną odzieżą" (or ""sklep z odzieżą używaną" the order doesn't make any difference),
btw, in Poznań (Osiedle Zwycięstwa) there was one store with a lovely name "Kup-ciuszek" which was a word game - it means "Buy an outfit", in a diminuitive form, but is also a clear reference to "Kopciuszek" (=Cinderella), which suits very nice 2nd hand clothes
Hi Jason, Put them next to the rubbish bins and something amazing will happen; you will see a hoard of down and outs arms out like zombies from those old 1970's B movies, and they will set upon your discarded undies like feeding time at the zoo.
The best thing is you will see them out and about for the next year! What better cause.