WWYD - mention mistake with gift or not?

[Dragontrainer]

A classmate of my son kindly gave him a WH Smith voucher for a birthday gift, which was marked by the child's mother as being worth ten pounds. Roll on a week and 8 yo DS is at the till with his lovingly chosen toy, only to be told the vpucher is for 10p. There is no way on earth the very lovely mum who bought the token would have paid for 10p and the lady on the till said that the cashier had probably put in the wrong amount when the card was bought. So, my dilemma is: do I mention the error to the mum? I don't know her particularly well and don't want to put her to the inconvenience of feeling obliged to rummage around for the receipt and to sort it out, but on the other hand I resent WH Smiths pocketing £9:90 that should be buying my son a horrible science kit! I

I would not mention it. She will be really embarrassed and, as the WHSmith cashier suggested, she did pay the full ten pounds in the first place and it was WHSmith´s mistake. I´m quite cheeky so I would probably have made a fuss in WHSmith and tried to ask them to correct the voucher´s value. I would say 9.90 is not worth making things awkward.