Amazon offers the Colgate Baking Soda and Peroxide Whitening Toothpaste 8-oz. 6-Pack for $7.92. Add it to your cart via Subscribe & Save to cut it to $7.52 and grab free shipping. (Walmart charges a few cents more via pickup.) That's about $2 less than what you'd pay in other local stores for the same quantity.
Be sure to cancel your subscription once your order ships to avoid being billed for further shipments.
Update: Subscribe & Save is no longer available; however, Walmart still charges $7.92 with in-store pickup.