TLDR : Answer Summary
The 18 countries that enjoy visa fee waivers in Thailand are the same ones that typically receive a 15-day visa on arrival. These countries include Bhutan, Bulgaria, China, Cyprus, Ethiopia, Fiji, Georgia, India, Kazakhstan, Malta, Mexico, Nauru, Papua New Guinea, Romania, Saudi Arabia, Taiwan, Uzbekistan, and Vanuatu. Additionally, there are discussions about the differences between 15-day visas on arrival and 30-day visa exempt entries.