fair play, the police are generally quite rubbish. But in the interest of getting your parcel back, have you tried contacting the charity to let them know one of their collectors has picked up a parcel which was clearly from the postman (with your name, address and paid postage)?
Also, have you agreed with royal mail that if you're not in, they can leave the parcel on your step? Usually they leave a card with all the information telling you where & when to pick it up UNLESS you've pre-arranged a "safe place" to leave the package. If you've not agreed for them to leave stuff sat on the doorstep, technically they're responsible for it being "stolen". If you have agreed for it to be left without a signature etc... then I guess you'll have to push the charity.
As for the police, ring them back and say you've just shot the postman & the charity sack thief. They'll be round in a minute
