I have donated to St. Jude in my sister's name (though she is still alive--I do it as a gift to her--long story), and they send her an email saying that I have made a donation, but not the amount.
When my mom died, we got cards from people who mentioned they had made a donation to XYZ charity (but not the amount).
So I say send the donation directly to the charity, and include a mention of it in the card that you send. Either a store-bought or a handwritten card will be appropriate.
DS '04 "Boogaboo"
DD '08 "Lilybear"