For Prime members only, Amazon offers the 20 Mule Team Borax Detergent Booster & Multi-Purpose Household Cleaner 65-oz. Box 6-Pack for $18.98. Check out via Subscribe & Save to drop it to $18.03. With free shipping, that's tied with last week's mention and about $6 less than you could expect to pay for this quantity in local stores.
Note: Be sure to cancel Subscribe & Save once your order ships to avoid being billed for subsequent shipments.