Keep in mind that polling using NPS is not an exercise in statistical relevance like traditional surveys are. While it's obviously important to extrapolate trends from the data you collect, the average response rate you'll get from an NPS survey (without sending a reminder) is well above what's necessary for reliable collection.
Detractors, also known as critics, are a kind of “taboo subject” in the business world - nobody really likes talking about or even thinking about angry customers. However, these customers are vital to business development. T
For example, Callexa Feedback can be configured to send a survey after an arbitrary number of days to the customers after an order has been completed or after a cancellation or after receiving an invoice. The customer can evaluate within this email. After the evaluation has been done, a website with a follow-up question or additional tags which can be evaluated, will popup.