With orders of $25 or more, Amazon offers the All Fresh Clean Essentials 30-oz. Laundry Detergent 2-Pack for $7.99. Clip the $2 off coupon to drop it to $5.99. Plus, Prime members bag free shipping. (Non-members can spend $49 to bag free shipping.) That's about $2 under what you'd expect to pay at a local store. It's sulfate-free.
Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.