@Tina713 you don't have any control over texas HOT (all done direct by abb), but if you typically add city taxes, you could adjust if their documentation checks out.
Refer them to AirBnB Customer Service. I am a huge softie but please do NOT offer them a kind-hearted exemption (ie, a discount) just because they say they are tax exempt or if they promise to fax you the info later.