Having been working with and advising companies on how to get great customer feedback for many years, I have a couple of ideas you could try.
One of the key assets you have is your customer list. You already have permission to approach them, I assume, so reach out to some to start a dialogue.
If you are able to segment them by sales, you can approach your great, repeat, customers with a simple question such as "What is it that you and your dog like most about our brand?" and "Are there items you are looking for that you haven't seen in my shop? If so, what?" or "What were you looking for the first time you found us?" Don't blast them to everyone, don't "survey" them, just ask the occasional probing question to try and understand what the "job" was they were looking to do when they engaged with you.
Another possible idea is a have a monthly contest where you ask a question in your newsletter and enter anyone answering into a draw for a small prize. Then publicize some of the best answers and give a shout-out to the winner in the next newsletter.
At the end of the day, you want to simply start a conversation with your customers, so you can listen and react to what they have to say.
There's only so much to cover in this forum, so please let me know if you have any questions.
Steve