You are not the first to feedback about such situations about Borderlinx, nevertheless, I feel for you as it happened to me before..... although not many times via borderlinx but the additional weight clocked by them sometimes is really quite sian, but bo bian since you can't do much about it...
It might be due to the packaging or the volumetric weight of your item or some other reasons. Check with them again?
What I did the other time, was that I weigh my item again when i received it in Singapore to see if it is really the weight quoted by Borderlinx.
I was unlucky to be charged more than what I expected via borderlinx on that ocassion. However, borderlinx shipping from US to Sing is really quite fast using DHL Express... so you win some, you lose some...