

Definitely not 300. And below the main price it looks like it says $14.44, so $20 makes more sense. Also I don’t think there’s enough space for both a decimal point and an additional digit behind the YouTube timestamp, so $20 seems the most likely to me.
It isn’t supported, doesn’t mean it won’t work ;)
They don’t claim support above 64GB because it hasn’t been tested and if it doesn’t work, you won’t get any warranty/guarantee/whatever for it. Meanwhile if your CPU doesn’t recognize 64GB or below, you may be entitled to a replacement or refund, for example.
In a similar manner, I believe that on certain motherboards you may be able to use ECC (error correcting) RAM with Ryzen 3000/5000 series CPUs, even if it isn’t officially supported by AMD.