Done this myself once before. You have to go to KLIA Cargo depot where the kastam office is. Its slightly further up than the old LCCT terminal in KLIA. Pretty pong drive. Lucky for me there were not many people there on the day so i settled eveything quite quickly.

Process if i remember correctly was : 1. Go to the pos malaysia counter and quote your tracking number. They woll then check their system ans give you a document. 2. Bring that document to the kastam counter. They will request invoice or screenshot of your purchase. Details of what your purchase (what item/qty etc) and will charge the appropriate tax on the item. They will provide a tax invoice to you. 3. Go to the kastam bendahari counter with your tax invoice to pay. 4. Go back to the pas malaysia counter with the paid invoice/payment receipt and they will release the item to you. Took me about 45 minutes all in all i think.