You are right that software as a services is a very gray area in the sales tax law. While many states have taken very firm stances on the issue, many rely on previously defined rules governing what is taxable and what is exempt in the state.
As it would related to TX, they are one of the states that does relies on the gray area of their law as it relates to software as a service offerings. For a complete analysis of the law, you may want to check the follow provisions: Sec. 151.009, Tax Code; Sec. 151.010, Tax Code; 34 TAC Sec. 3.308(b)(2); 34 TAC Sec. 3.308(b).
At a high level, TX takes the position that any software is taxable irrespective of its delivery method. They further go on to elaborate that SAAS is also taxable if it is deemed to be “data processing services”. There are several other rulings that I could point out for you but at high level, I do believe that the above would answer your question: SAAS and cloud computing are taxable in TX. If the vendor does not charge sales tax to you on the invoice, you should accrue and remit the use tax that would be due to the state.
The TX state rate is 6.25% percent but there are also local rates that apply generally making the total rate somewhere closer to 8.25%. There is a great sales tax rate lookup directly on the state’s website under the “Local Sales Tax” area where you can easily plug in an address and get back the correct rate for that would be due.
Here’s to hoping that this helps sheds some light on this gray area for and good luck as you move forward with this tough problem.