She cannot charge you the additional fee. If she made a mistake (a unilateral mistake) that would be something you could not have known would be a mistake, then she eats any additional costs. Further, you did not have a bargained for exchange or a meeting of the minds on the issue of additional monies for a quote, let alone actual work to decorate. You need to find out if this type of work requires licensure in your state and complain to that board. If licensure or certification is not required, then you need to complain to the attorney general or local prosecutor who handles or has jurisdiction over unfair and deceptive acts.