Can I book a return ticket for a 60-day SETV application if I intend to extend to 90 days?

For the record, I've applied for the SETV numerous times. The last two times, I included a letter (made a PDF files of my flights and letter) that stated I intended to extend and which Immigration office (in my case Soi 5 Jomtien). I also included extension stamp from previous travel. Never had an issue with the airline (just showed them my Visa), no problem entering (again showed my Visa) and extended within a month of the end of the 60 days.

You do have to submit a file showing a copy of your flights from airline, just telling them the dates is not likely to be enough.

Don't forget to apply for TDAC (you don't have to give them flight out for this) or use THIM app.

You'll need proof of onward travel within 60 days to satisfy your airline and Thai Immigration.

The 30 days extention of stay has to be applied for and is issued at the discretion of your local Immigration Office, it's not a given!

You need to check with your airline as to their requirements

Proof of leaving the country within the 60 days permitted stay is the expected information they require. Return flight is not required so how you do that is up to you if you don't want a flexi ticket. Airlines are the ones are most likely to stop you boarding if you don't have that proof. Other travellers may have never been asked but I have.

in the field "intended departure date (departure from Thailand)" you insert any date that lies within the first 60 days after your intended entry date. Don't upload any booked tickets, just tell them your "intended dates". In case the embassy wants to see real booked tickets, buy your normal 90-days both ways flight ticket, plus ONE cheap budget carrier ONE-WAY ticket out of Thailand within the first 60 days, like Air Asia from Hat Yai to Kuala Lumpur for 40 US Dollar. WARNING: do NEVER upload any "fake" onwardticket into the e-visa application. It is considered an attempt of fraud . . . about the bus ticket to Malaysia, I am not sure if the embassy will accept it. . . On a 60-days tourist visa, the airline normally does not ask you for an onward travel proof, but they CAN. In this case the cheap one-way flight would be the perfect proof