This post contains affiliate links. OzBargain might earn commissions when you click through and make purchases. Please see this page for more information.
Bonds has Buy 2 Get 1 Free sitewide until Cyber Monday. Tested on sale items and it works, and you can mix and match different products, the cheapest one is free. There's always free shipping without minimum spend.
At least for mens underwear, the same prices are available in-store at Target and Big W.