Goodwill/Salvation Army donations

TinaL99

We will be taking quite a bit of stuff to be donated in the coming months.  We've always just claimed the max. on our taxes but DH wants to keep better track of things this year and asked me to get a receipt when I take donations.  Do I pretty much just estimate what I think things are worth and give the people my guess and they give me a receipt for that amount?  I've never done that before so I'm not sure what to do.