I've always felt that people who say they are giving to charity instead of sending cards have got their ideas a bit 'arse about face' as my FiL used to say. It's not they who are going without, but their would be card recipients!
Surely a more appropriate gesture would be to ask people not to send them a card but donate the cost of card + postage to a charity of their choice. Then they would be the ones going without. In my experience, most people giving to charity instead of sending cards still expect and enjoy receiving cards 
Or alternatively, they should cut down on one festive treat - go without the bottle of Baileys for instance, and pass the cost onto a charity. But oh no, they don't want to make the sacrifice, do they? They just want us to go without their cards 