Sears offers the Ultra Plus 100-oz. Laundry Detergent for $6.49. Choose in-store pickup to dodge the $9.50 shipping fee. That's $7 off list and the lowest total price we could find. It purportedly lasts for 66 washloads under normal use.
Of note, Shop Your Way Rewards members will earn a Sears credit with this purchase and may be eligible for additional credits and discounts.