Didn't you clarify shipping costs and options with the seller
before placing the order?
In any event, computer cases are BIG and the boxes in which they
are shipped are even bigger due to the foam cushioning. I'd say
that S$88.50 to have it shipped from California to Singapore is
about normal for a package of that size and weight.
But why did you purchase a computer case from the US? Isn't there a
better selection in Singapore?