I bought some things over the years - usually skin care. 10% 1K discount came in handy.
One time several years ago I had a trip LAX-LHR & had to work the entire overnight flight because an unexpected presentation was going to be required when I landed/went to mtg, but my laptop battery wasn't going to last (this was pre normal outlets in the seat). I remembered they had one of those converter type things that plugged into the then seat power, as part of the duty free offerings. I talked to the purser, told him I was in a jam & really needed power before the normal mid-flight duty free, & he agreed to open up duty free once we created cruising altitude. He did, I bought it & worked the flight.
Ironically, while I was in the air the credit company thought there was fraud on my account because of the in-flight purchase, & shut down my account when they couldn't reach me. I found out when I landed & went to use the card at the hotel & it was declined. I was furious. Especially when I pointed out (calling them from the front desk) that if they looked at my account history, there were lots of flights & hotel charges on my card, so duty free would not really be outside the norm since it was a United charge.
Cheers.