Funerals right after the death are usually announced publicly via the obituary page, to save the family the trouble of sending things out, not to mention the close time frame. Memorial services are sometimes announced via the newspaper, and more recently via email and Facebook.
Your husband wasn't close with them. They may have wanted to keep the luncheon smaller and more affordable, and limit the number of people they have to talk to. They didn't want anyone coming out of obligation or guilt. She mentioned it to you but gave you an "out" - if you had been more insistent and said, "Oh of course we both want to be there," she might have sent you something. But you were more laid back (which makes sense because she kind of set it up with the comment that you didn't have to go), and between the 2 of you, it seemed they didn't feel the need to send an invitation. The distance could also have been a factor, plus the fact that you didn't know the deceased.
Let go of the guilt. You sent a card. That's fine. If you still feel the need to do something, you can make a donation to a charity in his memory. Those can include a reputable association dealing with the disease the man died of, his university, the public television station in their area, or the ambulance service that transported him to the hospital (if there was one). You can also check the on line obituary to see if the family named an organization for memorial donations. If you do that, include the names of the cousin to receive an acknowledgement from the charity; they will not disclose the amount.