Net Promoter Score is a metric that gives an external scoring of the quality of your internal processes. It measures your performance in the eyes of the customer. Like all other quality measures, NPS is only useful when kept in context.
For example, if you survey every shopper that did not purchase from you or every customer with a bad experience, you will have a negative NPS. Conversely, if you survey only successful shopper interfaces, you will get a positive score.