@Shelley *******
you would get a letter from your school stating you stop studying on the last day of your current extension of stay, MEET the requirements to get an extension of stay based on a Non-O (marriage to a thai, over 50, raising kids, etc) then go to the immigration office, cancel your current extension and pay 1900baht to get a new yearly extension for the new reason.
The ONLY reason I could see that you're being charged a "special fee" is because you're wanting to go on a Non-O extension based on volunteering.
What is the reason you are applying for a non-O? AND what immigration office is your current extension based on education from and what immigration office are you using to change reasons?