Sometimes I will think of a way to begin the conversation that doesn’t assume one or the other outcome. For instance, you could call and say how sorry you were to hear that the glasses broke before she got to enjoy them. You have some more on hand. Would she like you to set them aside for her? Then when she comes in, surprise her with half off. The key is that she will be so much more happy if she is surprised, than if you call and sheepishly offer a lower price. Shape the experience for max customer happiness, which will help you feel better about it too.