Amazon offers the Crest 3D White Lux Whitestrip 14-Treatment Kit for $30.99. Clip the $15 off coupon on the product page to drop the price to $15.99. With free shipping, that's the lowest price we could find by $17. These strips are designed to be used once a day for 30 minutes.

Update: The price has now jumped to $31.91 before the coupon, $16.91 after. Shipping info has also been corrected.