O’Conner screwed me over on Property Tax Protest

for 2022, I hired O'Conner & Associates after the appraised values were announced. Before the informal or formal hearing dates and before I could fill in home condition information and upload pictures in the client portal they had a sit down meeting with Harris county and got me a $5,000 decrease in value. Sealed the deal and sent me an invoice for payment.

I was furious and asked them why they didn't wait for the informal hearing or any of my data to be uploaded and they basically told me on the phone those services are mainly used by high value homes and they do the same thing for all the low hanging fruit.

I don't plan on paying their invoice which has been collecting 8% interest because I feel I got shafted and they didn't represent my best interest.

What is the worst they can they do?

PS: Also taking recommendations for 2023 property tax protest services.