It's only something my mother and I came up with last year, as I spent a fortune on sending cards two years ago. She thought of the idea and shes been sending cards back and forth between the US & UK for over 40 years and still didnt think of it sooner.
All Christmas cards for my family, including aunts, uncles and cousins (and their spouses) get sent to mom in one manilla envelope. I know she will pass them out as she sees people, and it saves an amazing amount. MIL puts hers in with mine, for people in my family she has met on various visits to the US. It ends up being about £3 to send them all. I've done it for three Christmases now.
Because it just works for us, we take Christmas gifts back with us during our summer visit, and my family gives us our gifts then, to save on shipping.