Amazon offers the Colgate Total Whitening Toothpaste 6-oz. Tube Twin Pack for $3.37. Add it to cart via Subscribe & Save to drop the price to $3.20. With free shipping, that's at least $2 less than you'd spend at a local retail store.
Be sure to cancel Subscribe & Save after your order arrives, or you'll be billed for subsequent shipments.
Of note, today only, Target offers it for $3.37 with free shipping when you buy it as a subscription and apply coupon codes "SHINE" and "BEAUTY".