Follow up…
Well, I went back to the shop because I was still getting some squealing, though not as much as was first happening. The mechanic and the owner both drove my car. They must have made some hard stops because everything in my trunk was knocked over! Anyway, they changed the brake pads to the ceramic type and the squeal is gone. The mechanic told me he put new shims on the car when he installed the first set of pads. He also said that the first set (which was something other than semi-metallic) seemed “really hot.” Everything seems to be fine now.
Thanks to everyone who responded!