Amazon offers the Charlie's Soap Laundry Powder Detergent 32-lb. Bucket for $108.60. Order via Subscribe & Save to drop that to $103.17. With free shipping, that's the lowest price we could find by $39. It is safe for all HE machines and allows for up to 1,250 loads (enough for almost three and half years if you do one load a day).

Note: Assuming you don't run a laundry business, be sure to cancel your subscription once your order ships to avoid being billed for further purchases.

Update: The price has risen to $110.83 before discount, $105.29 after. Still, that's a $37 price low.