What a bargain you snagged!
+1 on everything @GretchenS said. I too keep a small container of duck fat in the freezer. When I want to use some of the fat, I take out the container for a few minutes and then carefully (so I don’t poke myself) chip out a portion with a sturdy cheese knife.
My favorite use is for roasting potatoes. Sometimes I squeeze a lemon wedge over the finished potatoes, though it’s hard to improve on the heaven that is roast potato, duck fat, and kosher salt.
