Amazon offers Prime members the GoPro Floaty Backdoor for $11.67 with free shipping. (Non-members can spend $35 to qualify for free shipping; otherwise, shipping adds $5.74.) Outside of the deal below, that's the lowest total price we could find by $6. Compatible with all HERO3 and HERO3+ cameras, it includes a float backdoor, two adhesive anchors, and two camera tethers. Note that this item may take three to four weeks to ship.

Can't wait for it to arrive? Heart Rate Monitors offers it for $14.24 with free shipping via coupon code "010711".