If you're really worried about it, note that there are 4 manufacturers whose products and manufacturing process have gone through the official ISO testing and certification for safe solar viewing: Rainbow Symphony, American Paper Optics, Thousand Oaks Optical, and TSE 17 (more info at https://eclipse2017.nasa.gov/safety )

I got my eclipse glasses, and my camera solar filter, from Thousand Oaks. Good company, been in the business a long time, has had previous large-scale contracts for their solar filters with NASA, Discovery Channel, Nat Geo, etc.

There are probably other manufacturers that build perfectly safe solar filters but haven't put in the time to get officially ISO certified. I'd expect that most eclipse glasses on Amazon are probably just fine. But if you're worried at all about it, go with one of the previously mentioned 4.

As an aside, is that $9.99 for a single pair? You can get packs of 5 or 10 for about that much from other manufacturers. American paper optics has a 10 pack here for $14. Thousand Oaks has a 5 pack here for $12.