Crest 3D Whitestrips Glamorous White, 14 Treatments - $38.96

I've been tracking these strips on Keepa and they just went below $40. The last time that happened was for a couple of weeks last year between October and November.
They are available at $38.96 after the $1 coupon on the product page.

I have used these so can attest that they work quite well.
That said, this is the stronger version of the whitening strips so YMMV if you already have sensitive teeth.