appleegg,
The fee is US$100, which used to be 4,000 baht but the baht has been pretty strong for a while now so the baht equivalent may have been adjusted. (It's adjusted by the consulate only periodically - doesn't vary daily like the exchange rate at a bank.) But plan on roughly $100.
You can pay the day before the interview - as long as it's a day that the post offices are open. Someone got caught by that a while back, didn't realize a Thai holiday or something that closed the POs the day before his fiancee's interview. I'd suggest paying a week in advance to allow for the unexpected, being careful to not lose the receipt.
Yodrak
QUOTE(appleegg @ Sep 8 2006, 06:36 PM)

after receiving a packet 4 and an interview date for a cambodian fiance at the bangkok consulate, when and how do i make payment? i understand i can pay at the post office, how much? and can i pay the day before the interview? if not, how far in advance must i do this?
mahalo to all.