My own personal experience is to deal with the airline directly and not use travel agents, rescheduling can be cumbersome, difficult and expensive. One way purchase is usually less expensive to the destination than the return because of the early date, example booking 10 days before the flight could be 25000฿ more expensive than booking 45 days in advance. Choosing the the flexible date option on the return with an airline can be very expensive, not only on the upfront end, but when you rebook the return they’ll charge whatever the current price is for that flight on that date. So it’s important to speak with the airline to get a clear understanding and be sure to read documented agreement of the fair base type of the ticket before purchasing to avoid any misunderstandings. Scout out the price on the internet then call the airline.
If the international airline doesn’t provide booking from home city, then book separately one way from home city to international airport and book the return to home city about two weeks before the return, because of the frequent schedule changes of the US airlines.
Of course, booking round trip and not changing the schedule will always be less expensive.