I don't particularly mind showing my boarding pass. I have it with me and I'm probably not going to be militant at the checkout, but I'd prefer it if airport shops passed on extra savings to me rather than pocketing it.
I won't buy anything other than water and a newspaper - which often comes free with the water - in WHSmiths.
I hate WHSmiths with a vengeance, but it does seem like cutting off your nose to spite your face to get to your destination and have to rush out to a supermarket to buy water because you hate WHSmiths.
The only other thing I routinely buy is make up, but I know what I want and know the prices and will only buy things that have savings. MAC eyeshadows and lip glosses mostly, they're good and I'm sure there are other brands.
I'm a bit of a sucker for the summer versions of scent too that you always get at airports. My niece bought me a lighter version of Calvin Klein's Eternity that was lovely.