Actually Singapore customs state the $400 is calculated via this method
Cost of item + [(cost of item + freight ) x 0.02 ] = 400 and above , will incur GST.
Which usually works out to if your item is around USD$250 and above, be prepared for GST.
In Singapore, just make sure you have some pocket change ready for the UPS/FEDex/Singpost/DHL people, cos they accept COD only.
Else they'd come back another day.
And for UPS/FedEx/DHL a $10 forwarding fee is included on top of the GST if GST is applicable.