Best Buy offers the GoPro Floaty Backdoor for $10.99. Choose free in-store pickup to dodge the $1.99 shipping charge. That's a buck under yesterday's mention and the lowest total price we could find by $3, outside of the price below. It's compatible with all HERO3 and HERO3+ cameras, it includes a float backdoor, two adhesive anchors, and two camera tethers.

Not in a rush? Amazon offers it to Prime members for $11.67 with free shipping, but it will take 1-2 months to ship.