Adii's answer is a great one.
I travel alot so I like to invite customers to a lunch or a coffee if I'm in their area to find out if we can help them in any other way or to hear feedback on what they don't like or what we can do to be better. Having them involved on a meaningful level where you can share your vision with them creates the most meaningful and valuable relationship you could hope to have with a customer. It goes a long way and creates evangelists for your brand. That's ultimately why you want to delight your customers.
I recently just sent this email to Steve at Kontagent, one of our customers, as he was a customer reference for an investor in our company:
Thanks again for the time with Eric. We're gonna know Wednesday on their decision. They said it went well and if we get approval, I'll be in town the 6th or so. I'd like to buy you lunch or a coffee if you're interested.
And as good as your feedback has been, could you tell us what you don't like about Ecquire?
Thanks again, Steve and let me know on both items.