Amazon offers the Amazing Grass Green Superfood Drink Powder 8.5-oz. Container for $19.99. Clip the 20% off coupon and order via Subscribe & Save and it falls to $14.99 with free shipping. That's the lowest total price we could find by $8. It's vegan and gluten-free.

Of note, be sure to cancel Subscribe & Save after your order ships or else you'll be billed for subsequent shipments.