If I'm not mistaken I've seen this product being sold by several less scrupulous kava vendors. I specifically remember the pictures of the product in a measuring cup with words written into the powder itself. In fact, I believe kona kava farm also sells this. It could be the source.
Personally I'd be wary of any product marketed as kava extracts unless they've been tested. However, we simply don't have any data regarding this product, so we can't say with certainty if it is legit or not.
@Ethan, what some vendors will do is purchase "kava extract" from China (which I've seen and it doesn't look good at all), add that to their micronized kava, and call it kava extract powder. True kava extracts wouldn't be in a powder. Kavalactones are waxy and make more of a paste than a powder. 70% kavalactone extract would be a thick paste.
Regarding your question, I use them by sublingual absorption or just eating it like peanut butter. I love paradise kava's extract. The only issue is they're too good, and tend to run out....very quickly.
Edit: Also I've made my own instant kava before with kavas that were really high in kavalactone content. Even after I put the product in a vacuum oven for drying it would still stick to itself slightly due to the kavalactones sticking together, so powdered extracts scream questionable to me.