I have a 90-day multi-entry F visa. I entered the country on June 5 and am now getting ready to leave again. I found a super cheap flight to seoul leaving September 3 and I'm unclear when is the last possible day of my departure. Can I pull it off? Also, how do you buy your international plane tickets? I found the best price on eLong compared to qunar.com and ctrip.

Thanks in advance!