Personally I wouldn't make a thing of it at all, cards only is just fine!
My grandsons send me lots of lovely letters and texts, but they still only get a card, they know if I could afford presents they would get them, just as I did it the whole time they were growing up.
Honestly, you don't need to justify it, you have always sent gifts, now they are adults, and I am sure it won't cross their mind if they only get a card.
I print my own cards too lol, I call myself stingy but the huge family I have it costs a fortune to send something that will be thrown away the next day! :-)