Many businesses want to receive feedback from their customers to allow them to improve. It may be that they want to know how well their website is working or if they could offer you any other features that would make the ordering process easier. They may also be interested in knowing how you would rate a product or service they supplied. Some websites do this with great success but other companies struggle to get much response from surveys they send out.
When emailing a customer firstly be sure that you have their permission to do so. When gathering their email address, make sure that you tell them they may be contacted from time to time but that you will not share their data and always allow them a way to opt out.
Create your email based on the most important information you need to gather. You need to ensure that the process is quick and easy and if possibly offer them an incentive for completing it such as money off vouchers etc. Keep your questions short and to the point and offer them a multiple choice selection as well as a comments box should they need to express their views in more detail.