Customs check the invoice, then add cost of postage, then add insurance if you had any and you get slugged the 30% on the TOTAL price. I tried to reason with the customs guy that I shouldn't be paying duty on the postage. He was very polite and friendly, almost apologetic, but said sorry but that's what's written on his instructions.
Re lap tops, my last trip back to LOS I bought in 2 lap tops, wasn't searched so dont know if I would have had to pay anything. In the Bangkok Post, Database (weekly computer) section, they say lap tops in Thailand are no dearer than in Singapore. All Singapore has to offer is a wider variety, and the only the really top end lap tops may be a bit cheaper, but all the normall everyday ones are same price in LOS.
I see in Makro in Phuket, Compaq lap top, 80GB hard drive, 14" screen under 20,000 baht