Just admit you can't be arsed to send any.
And is the charity you are supposedly donating to, going to go wild about the 1.99p it would have cost for a box/pack of 50 Christmas cards? 🙄
I suppose some people will come on here and say they send at least 2 dozen cards out by post/abroad etc, and that costs them £50-60 in postage, but most people don't do this. Most people just send them to neighbours, close family, acquaintances, and work colleagues.
As I said, just admit you can't be arsed, and bore off with the sanctimonious 'I am not giving out any Christmas cards this year, I am giving the money to charity' line. (I bet most people who say this, don't even give anything to charity 'instead of sending out Christmas cards.') 