I am trying to buy extra baggage allowance with ThaiSmile - I can find the details of how much it is but no place online to purchase it. Nok and AirAsia all have the extra baggage purchases when you book the tickets but ThaiSmile does not have that option. I have looked at the FAQ and it does not tell you how to buy it. Anyone know how?

no sorry I don't know, but why don't you just call them ?

indeed the good thing about Budget-Airlines is that you can easily add luggage allowance at your convenience, whereas big legacy carriers usually don't have any such option - and THAI SMILE is not a budget Airline after all, it is just a regional offspring of THAI AIR INTERNATIONAL, some sort of hybrid model. or let's rather say, they still until today actually don't know what they want to be and have changed their business model and route plans more than once in the short time since they started operations.....

Guess: pay at check in.

https://www.thaismileair.com/en/BaggagePolicy

Excess Checked Baggage

Passenger is allowed to carry luggage weight not more that stated in section 7.7 free of charge. Passenger may not bring baggage weighting more than the threshold that allow transport without any charges. If, in the company absolute discretion, baggage in excess of the free baggage allowance is accepted for carriage, the passenger shall pay a charge for the carriage of that excess baggage at the rate of THB 100 per kilogram for domestic routes inclusive of taxes and THB 300 per kilogram for regional routes.

I found no method to prearrange excess baggage in the booking process.

Check in will write note how much to pay at Thai Smile desk within nearby Thai ticketing office. Boarding card given when proof of payment handed over at check in desk.

Went trough this some 10 days ago.

I'm gonna check my emotional baggage at the boarding gate..

Then run like hell in the opposite direction!