If nothing else, please ask your customers why they are leaving/cancelling. It makes me feel like a company doesn’t care and won’t get better if they don’t ask leaving customers why.
I probably don't hate you, but if you make this as an interruption to me cancelling, I very well just might.
Also, I think having just a free-form text box is best. The text of options in the radio list of reasons tells a lot about the mindset of the company. Quite often, not the nicest things.
That way your cancel's done and dusted but with the option to feedback if you fancy it
I've seen a few sites that don't make it clear that you have successfully cancelled and act as though the survey is still required, so don't do that. Make it clear the user has finished cancelling and this is an extra step they can take if they'd like.
Caring is important, even if you're caring about your business in the grand scheme of things