Sounds like a great deal .I could also spend $40 on an Uber to go to my nearest (reputable) LFS to get some coralline algae, so $17 to have some delivered sounds good to me.
but why isn’t it already growing ?
Chances are you have already introduced it somewhere or somehow .
if the system isn’t stable enough to grow it without adding the magic bottle, why would the magic bottle of essentially the same thing work ?