Bonds underwear hipster brief 5 pack for $21 delivered. Seems to get pretty consistent happy reviews and for $4 a pair you could surely do worse.
You can combine with the $5 sign-up voucher BUT you lose free delivery if you go under $20 - so maybe throw in a $5 pair of underwear and you can end up getting 6 pairs for $21, with the voucher.
Shopback has 13% off but I doubt that would work with the voucher.
Women's underwear are also on discount but, due to factors beyond my comprehension, seem to be more expensive than mens.
member since 2008 and that's your title ?