These things always happen to me.
Well, it's about vegetable getting mixed up but same problem.
If you think she was upset because she understood it was 20 (and that was actually the price) and she had to pay 40.. I think it's better to contact her, reimburse her and tell her there was a miscommunication between you and your mother.
In my experience, corrected errors, when the customer can see the vendor is extra honest, have the same consequences (or even better!) than no error. Customer become more trustful.
If 40 is the price, but you think she may be pissed about the lining.. maybe offer a small discount on the next one?
In any case, for the future you should propose the price yourself and do a basic written order in double copy. It's saving you from a lot of trouble!