YMMV double coupon method. Amazon has the 4-Pack Colgate Total Whitening Toothpaste Gel (5.1oz each) for $15. Here's how to drop to $9.90. 

1. Click the one-time purchase option, click the $1.08 off coupon.
2. Click the S&S option, click the 20% off coupon.
3. Checkout with the 5% S&S discount.

Should look like $15 - $1.08 off - 20% off - 5% off = $9.90 with free shipping on $35+ or with Prime. Roughly $2.48 per tube vs. $3.98 at Walmart.