Yes, you pay based on weight. You can also get special prices on certain items. Like a vacuum cleaner will only be $3.99. Big items have a set price - they have things like furniture sometimes. If you find stuff that is heavy, you can make a deal too. I have had one item cost $20 by weight but I asked for a deal and they sold it for $9.99.

It's not too common. The Goodwill Outlet is a special place where unsold items get one last chance before being shipped off to who knows where. It used to be China before this stupid trade war stuff started up.