In my experience, about 20% of the stuff I buy arriving through EMS or even ordinary post gets pulled up for GST. It does not matter what is declared on the shipment, once pulled, you will be asked to provide documentation to prove its contents and value. I have had packages marked gift and less than USD100 that have got pulled, while packages fully insured for USD2000 slip through. The most "painful" episode was for a Birkin you can imagine what the GST on that was!
As sweetprincess pointed out above, it all depends on your luck (or not). You do not have to go to Paya Lebar to get your shipment, you know. Just fax in your documentation and pay an extra $15 (on top of the GST) for home delivery or delivery to a nearby post office.
The safest way not to get taxed is to pay for shipping through FedEx (or DHL or UPS). You pay extra for shipping, but FedEx will only charge GST (on behalf of the government) if the declared value is above SGD400. They do not do random spot-checks, unlike SingPost. You could ask the shipper to under-declare the value, although technically, this is cheating and you could get prosecuted for evading GST if you ever get caught. My philosophy is, if you can afford the bag, you can afford the GST. If it slips under SingPost's radar, more power to you; if not, just pay up.