in order to qualify for free shipping, it must be sold BY Amazon themselves. There is a difference between sold BY Amazon and sold VIA Amazon.
Items sold BY Amazon can qualify under Amazon Global that provides free shipping to SG if items over a certain amount. However, third party vendors selling using Amazon platform (u can see the seller details via the link at the item page) will NOT qualify for free shipping.
Nonetheless, i still use Amazon for hard to find items in SG and when factored in the shipping fees, i found many items still cheaper. Of course, items like electronics that run the risk of breakdown and need local warranty, i will think twice then. Just like what happen to my amazon shipped harmon kardon speakers that spoil less than 1 year.