San, N. E. E., and M. Aryupong. “Effects of Sales Promotional Tools on Product Quality, Brand Image, and Customer Satisfaction: The Application of Inverted U-Shaped Theory”. Business Review Journal, Vol. 12, no. 1, June 2020, pp. 84-103, https://so01.tci-thaijo.org/index.php/bahcuojs/article/view/176208.