How can I book a round trip flight while planning to extend my stay in Thailand from 60 to 90 days?

Bangkok, Chiang Mai, Phuket, Buriram and other offices allow you to apply for a 30 day extension when you have 45 days or less left on your current stamp, most other immigration offices let you do it when you have 30 days or less left. There's no reason to wait until your stamp is almost expiring before you apply for the extension because the ADD the 30 days to your current expiration date..

@Tod ********

thank you

the airline they fly in on isn't going to ask them for proof of onward travel because they will have a valid VISA to thailand in their passport.

As a rule the airlines only ask for proof of onward travel when you are flying here without a visa in your passport.

We are US passport holders also. We bought our 60 days tourist visa in NY and extended for another 30 days here in Chiangmai with no issue at all.

Yep, no reports of immigration offices not issuing 30 day extensions on a 60 day tourist visa entry.