25 quid (don't know what cards cost in UK) for cards, plus any postage, to give the impression,(as Vin says) like your begging for a tip. er, nah. I'll leave it thanks.
They're customers, not my friends. If they don't pay, they become enemies, so a card at Christmas is a bit over the top for me.
Think I only send about 5 cards to anyone at all. Bah Humbug etc.