I just did the exact same thing yesterday for another product I buy, not online, but in a store. Without getting into why I was becoming dissatisfied with their product, I nicely said I wanted to make them aware their quality was making me think of changing products.
They replied within an hour and said they already knew about the issue and were working to resolve it and offered to send me free samples of a comparable product to try in the meantime while they worked out the issue.
One time I commented on a change in my favorite frozen pizza and they thanked me for my comments and sent me coupons for free boxes.
I think they like and want the feedback good or bad, I would.