Your case is different than most. The airline should collect 700THB due when you exit Thailand, but they may not. I've been able to avoid paying it most times. I just tell them that the ticket was issued in Thailand and I'm sure the fee was properly collected.

gleff was correct, you're aplicable to pay it if you've entered Thailand.

http://www.airportthai.co.th/airport...viceCharge.pdf

It's important passengers be carefull not to be overcharged/doublecharged.

Example
If you fly domestically to BKK on a portion of an international ticket and leave the airport instead of an immediate departure (no matter if it's 2 hours or 2 weeks), note that in most instances the appropriate 700THB tax has already been collected when you ticketed.

If upon check-in for your international flight, you're asked to pay the fee, remember the only extra fee you could possibly be responsible for is the domestic departure tax (It's 100THB IIRC).