This GoPro Pro 3.5mm Mic Adapter (HERO6 Black/HERO5 Black/HERO5 Session) (GoPro Official Accessory), almost never goes on sale. It's almost always selling at full retail rip-off price of $49.99. Unfortunately, for those that want to use an external mic (e.g. Rode VideoMicro) with their Hero6 Black/Hero5 Black/Hero5 Sessions, this USB-C to 3.5mm mic adapter is required (due to the built in DAC).
Currently, there's a $10 discount at Target for $39.99 (+tax in most states)