Matt, people forget you fast online. They even forget what they have purchased the day before.
You have to remember they are not just on your list.
They could be on dozens of other lists and marketers are mailing them every day.
You can't expect everyone to remember who you are even if you make it clear and concise to them when they sign up.
Human nature is such that people do complain. Some people are never happy, some are forgetful and some you will never want to do business with again.
That is the price of doing business and you have to develop a thick skin or you will doubt yourself/moderate what you are doing based on a tiny percentage of vocal people.
I once did an experiment by replying to anyone that unsubcribed from my list. I addressed whatever it was they wrote about when they left a comment.
Do you know how many of those unsubscribed replied to my personal message?
Now I just delete them without even reading it's not worth your time. They are off your list - good, they were never going to buy anything anyway.
If someone complains they are removed from your list automatically so do not sweat it.
My 2 penneth!