all stores in funan do that, as long as u ask them to print you a tax refund receipt..
that aside, u gotta be a foreigner and flying off from Singapore within 60 days of your purchase..
been there done that.
IIRC, you can get a 'third party' refund which gives you 5% immediately at departure or the full 7% refund but you have to jump tru a few hoops and wait a while. Check the IRAS website for details.