I am certainly not a property tax expert (and don't live in Texas), but I would believe that the definition of commercial property isn't whether you let people in the front door, but rather that your product is available for sale to the public. Don't confuse "retail" with "commercial", retail is definitely commercial, but commercial is not necessarily retail.
I have worked for commercial enterprises my entire life, and never had one that allowed the general public in the front door. That didn't make them not commercial, it made them not retail.
When you got the building permit did you tell them what you were going to do with the space? Did you let them believe it was a garage or personal space? Pretty hard to keep a business a secret from your neighbors. And one or more of them might be saying "Hey, that land isn't zoned for a business!" And the law is probably on their side. If you thought you were going to "get away with it" (knew better) than you can't complain when the law catches up to you.
Personally, I would let them in the door, take my lumps and move on. The alternative could be far worse.