Calling all spouters.....Are prices negotiable in thrift stores

Villas - Calling all spouters.....Are prices negotiable in thrift stores that donate to the needy? My guest from the South offered 15 for a 20 dollar item in a store and a man jumped up from behind the partition telling him he was cheap, prices aren't negotiable because the money goes to the needy. Is this right? The non negotiable thing is bugging me.