GST. Smartwatch? SIRIM as it is a bluetooth device 

Yes only pay GST. No import duty for regular watch  You appointed courier company would usually pay GST on behalf and charge you the amount they paid + service charge, unless you have chosen other option for custom clearance

Same thing happens in US where USPS usually get a pass while UPS or Fedex will get import tax going in US.

The problem is buying from US, some time the shipping fees is more expensive using USPS, up to $30-$50 USD more.  My guess is USPS works conjunction with POS Malaysia while others not. If you want to get POS Malaysia delivery domestically, you have to pick partnered carrier on foreign soil. So there's uncertainly if you take the chances of POS Malaysia, possibility avoiding tax but having to pay higher shipping cost. in my case, fedex is better deal since i already save about $30 USD in shipping, only to pay $60 RM in Tax.