You can get all the information and help for free from BOI; look at their website; the total costs are 50.000thb to pay directly to immigration office or to the embassy (there might be a difference in this amount when abroad). It’s a waiste of money to pay more; if you meet the requirements it’s a piece of cake.
You better ask your immigration office; then you know as well the exact amount what it will cost, should’ve be about 5700thb for extension and multiple entry
You were lucky, but you post it here and other people may think that this is normal procedure but it is not!! The airliner can refuse you on the flight when you don’t can show that you will leave in Thailand before the end date of your visa or visa excemt period