I think that both you and her writing the store a note would be a great thing. And even though it feels like emailing the head office is a little thing, trust me, it isn't.
Where I work, there's a discount if you are over 55. Obviously, this is a rather delicate point, and can be hard to explain without offending. One woman was impressed by the way her clerk handled it and emailed the HQ. It resulted in a company-wide video "memo" on how to gracefully handle the transaction, using that clerk as a model. One customer thank you can mean a lot.