thank you all! i will be trying this CGW!
Well just an update here(on my own questions).
I ended up shipping all my items in 2 separate shipments, total shipping fee came up to USD84 without GST.
If I did combine all of them, they would cost USD 95 with GST inclusive, so well CGW gets more money, IRAS gets nothing
Also, even with the weird calculations from CGW, I managed to play around the system, and using the rate I see for USD (1.25+/-), guess the safe zone is to keep the shipment+items prices below USD280.
=) Thanks again guys! Using CGW free express (from Amex), waiting for the shipment to come in next Monday.