With orders of $25 or more, Amazon offers Prime members the ComStar Pure Lye Bead Drain Opener 1-lb. Jar for $5.30 with free shipping. (Non-members can spend $35 or more to qualify for free shipping.) That's the lowest total price we could find by $7. It contains 99% pure lye beads to clear drains, grease traps, septic systems, and cesspools.
Note that this item is offered as part of Amazon's "add-on program" and can only be purchased with orders of $25 or more.