This just happened to me (not my first lost luggage, but the first time that it was not dropped at my house while I was working). UA lost one of my car seats and gave me a loner car seat. The next day I was called to the valet section to get my car seat and exchange the loner. After we traded the delivery guy just stood there. I didn't see the point of tipping since I would not have tipped someone if it arrived on time with the flight.