Rate quotes on avisthailand.com do not include VAT . Also, the EU sites all include zero-deductible CDW insurance, whereas on the Thai site the CDW deductible is 8k or 15k THB depending on car class, and the zero-deductible option costs extra. That may well be the same with the US site and explain a good chunk of the price difference.
I`ve used the Thai site for a rental at Krabi airport once in 2019. No issues. Almost new car.
Avis Thailand does not seem to participate in the US or European loyalty programs. My status didn't get me any benefits or was recognized in any way.