I manufacture and sell anti-fog inserts for the GoPro cameras, similar to what is sold by them, but at half the price and with more inserts per package. The material is exactly the same (raw material from the same manufacturer), so expect the same results and quality. The only difference is that I am a one person "company" - maybe two when my wife helps out, so my overhead costs are zero, thus I can even beat the mass volume costs of GoPro's manufacturing and sell it for less.
Each package has 4 pairs, that is, 8 inserts for the Hero1 and Hero2. Since the Hero3 housing is smaller, the inserts can be cut in half to fit the new housing, yielding 16 inserts.They can be re-activated and used countless times.