That's fine, as long as you're going to deliver them by hand.
If you can find a local stamp shop that sells "kiloware" (unsorted stamps on paper), or an on-line advertisement for the same stuff, and you compare the selling price with the cost of even 2nd class postage, you'll realise that POSTING used stamps to a charity is a complete waste. They'll do far better if you put the stamps in your waste paper, and send them a donation of the amount that you'd have spent on posting them.