I agree. The most important thing to Thais, second only to saving face, is money. Once they have it they have it and you have a heck of a job to get money back for defective goods. Credit card slips (not sure if anyone signs one anymore) even said, in capital letters, NO REFUND. But if you stand your ground and argue without being abusive then you are likely to come out on top. They can give proper and professional service, but only if they want to and you make them feel extremely uncomfortable.
On this subject, I once read on this site that a major store refused to replace or give a refund to someone who returned a pair of shoes he bought the day before and then the sole had fallen off. They refused a refund, he refused to leave and they called the police. Customer service, Thai-style.