Made a purchase through duty free on a Lufthansa flight a few days ago; this was interesting because their reader had a slot to dip but the F/A swiped and it went through -- so no service code enforcement (receipt shows "Method of Entry: S").

Also interesting (and I'll xpost this into the DCC thread), a DCC offer came up after she swiped and she didn't even ask me about it, instead immediately picking to charge me in EUR. That's the way I like to see DCC.