I tend to wonder if your DD was pleased with the compliments about her bag, and in the heat of the moment said something like "My mom loves to sew and I'm sure she wouldn't mind making a bag for you." and then did not know how to tactfully add that there would be a cost to the process.
I guess I would do it this time but make sure that you DD and this lady understand it is a one-off and any further items would carry a price tag.