Although it does annoy me I have to pay to attend, at least it goes to charity. But we don’t get a tax receipt so I assume the boss claims everyone’s donations on his tax as we pay through Eventbrite.
In previous jobs there have been events (never a Xmas party) where they would encouraged us to donate by giving us raffle tickets for every $10 donated and someone would win a prize. We would always directly donate ourselves to the charity, get a tax receipt and then email the donate confirmation to receive our raffle tickets. It made you feel good about donating and we would actually raise a lot more money this way.