I find it surprising that you only had one "stop spamming me" reply. I've cancelled two services that have sent similar mails, and I figured I wouldn't be /that/ unusual.
My assumption being that if you're sending unsolicited mails so quickly, then I should be expecting a truckload to arrive by the end of a year. In one of those cases I felt vindicated as I received a bunch of huge exit interview type replies.
My bad, that was an uncharitable way to phrase it. I didn't mean to imply that you are filling inboxes, but more that I believe it tilts the balance in that direction.
"We are very interested in how you are getting on with <product>. Please email us if you have any suggestions about how we can improve <product>."
But that only works if you are getting sales.
Also the more responsive you are to customers, the more likely they are to report bugs or make suggestions.
I wrote about my experiences getting feedback here: https://successfulsoftware.net/2008/04/28/getting-customer-f... (old article, but still relevant)