In my opinion, what you are doing is absolutely critical. If I am offering feedback, whether it is positive or negative or a little of both, the most important factor to me is knowing that someone cares enough to respond.
The process is dependent upon the infrastructure of your organization. It sounds like you are the only person who is seeking, and responding to, feedback. If so, do you have the ability to follow up on a follow up? For example, if the complaint is about the length of time it takes to process an order, are you able to follow up after their next order to see if they are now better satisfied? What happens with the feedback that you receive? Do you pass it along to those who need to know? Do they want to know? Does your organization track issues and make changes to its processes/policies/procedures to better serve your customers?