I've never made a custom order in Thailand but wonder whether they could give your approximate quote, ie +/-10% before you place an order.
That way, you will know whether you may want to order directly by yourself or via them.
I also have this problem lately with amazon.com too. I placed an order for a few RCA Living Stereo SACD @ US$11.99. Someway somehow, they haven't sent it after 1 week so I cancelled the orde.
I then placed another order, making sure all the thing will be delivered next business day. At the time, the price of the same SACDs dropped to US$9.99 each. They were all delivered next morning.
I checked back the SACD price again one day later, they are all backed up to US$11.99... Just a very narrow window of opportunity to get almost 20% less per SACD. Pure luck, I guess.