How to deal with those customer who complain about expensive wines and champagne?

I assume we are talking about restaurant service? If so, then that's why it's so important that a restaurant serving expensive wines and champagnes have a trained sommelier/wine steward. Someone adequately trained to describe to the customer what they are getting and to suggest appropriate food/wine pairings. Also someone who, if the customer doesn't like the product, can sample it him/herself to see if it tastes as it should. If the product is "off", then the customer shouldn't be expected to drink it or pay for it. If the product is in good condition, then it becomes a judgement call, usually by the manager, taking a number of factors into consideration.