There was a story in the news a few weeks ago, that when airports retailers demand to see your boarding pass iat the til, it is just a VAT scam from which they make a few bucks. Apparently it's not mandatory at all to show them your boarding pass.
The fact it is a VAT scam doesn't really bother me, but I detest having to fish around for it in among the travel documents. So a few weeks ago I put it to the test, having forgotten to pack a Euro adapter, and resorted to buying one at Dixons.
Shop assistant:
May I see your boarding pass?Me:
No.
Shop assistant:
OK.