You DO NOT have to purchase the $15 of worth in the same trip. It just has to been done between now and August 8, 2012 in how every many trips you make it.
Here are some available coupons for Wisk that are out there that you can use to help make this a real steal!
$1/1 Wisk 45oz 06/10/2012 RP
$1/1 Wisk (26 Loads or Larger) Printable