I took an UberX from SEA to downtown Seattle and my receipt showed a $0.37 "per-minute wait time fee" in addition to the upfront quote.

I had stood in the airport pickup area, waved at the driver as he came down the ramp, and entered the vehicle right after it pulled into the parking space. I made no stops en route. I had no luggage, so no time can be attributed to even opening and closing the trunk.

So either $0.37 is a hidden fee for all SEA pickups, or my driver started the meter prematurely.