Doctors send recall cards out to remind patients it's time to give them a check up. Cars needs that too.
A classy considerate garage sends recall cards to their clients. Oil changes are needed so the car can operate better, wait, why am I tell you this? You should know this. Anyway, if they are on your mailing list, send these reminders out. Chances are they probably forgot and it needs the change.
Also note that you can easily change the message to anything else, like a recall, or some other service. A thank you note is all commendable.
Perhaps they will remember you in the future, all because of this little reminder card.
Be sure to remove the Address on the left page, and replace it with your own including your phone number.