ok...here is what i roughly work out...
if his friend stays in a state that no need to add sales tax:
cost for 3 pieces = US$181.95 * 3 = $545.85
after 10% discount and $40 coupon = US$451.27 (saving of US$94.58!)
shipping within US is free, so each card will cost US$150.4 or $263.2
However since total is more than S$400, need to add 5% GST, so each card will be S$276.4, but we still have to share the cost to ship the card back to Singapore
if his friend stays in a state that need to add sales tax, need to add 8.25%
that will be US$488.49 for three cards, or S$284.95 each, add 5% GST will be $299
worse case will be using vpostusa, still need to add 8.25% tax and 5% GST, but vpostusa will charge higher for shipping