To answer the original question about "shipping a case of wine with goods" - there is no duty-free allowance as you are not bringing them in on your person. You will need to tell the shipping company and make the customs payment (its not difficult to do).
You can bring in as much wine on your person when you fly. The duty free allowance only goes to 2l the rest is dutiable. I usually purchase at the bottle store and then send the wine through checked baggage (you can't bring it onto the plane with you because of the liquid limitations). When you get to SG - just tell the customs officer how much wine you have and they'll calculate and make you pay duty. I've found that they wave you through if you have 2.25l of wine (3 bottles) - they can't be bothered with the paperwork for 250ml.
