Amazon currently has a 2-pack of the Arm & Hammer 210oz commercial sized laundry detergent for $18.88, or $17.94 if you have 5 S&S items.
I almost passed over this when I first saw it until I realized that it is a 2-pack, so 420oz/280 loads for a really good price. It works out to about $.067 per load or $.064 w/S&S.
https://smile.amazon.co