Exclusively for Prime members, Amazon offers the Colgate Optic White Platinum Express White Toothpaste 3-oz. Tube bundled with a $2 Amazon Credit for $2 with free shipping. Assuming you'll use the credit, that makes it essentially free. (It's also around $5 less than you'd pay in local stores, after factoring in the credit.) It purportedly gives users whiter teeth in three days.

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Note: The credit can only be used towards purchases of select toothpaste. It's also limited to one credit per sample per customer.