I don't recommend mixing them.
I did it just for fun and it's isn't really very effective.
As for buying NPA and PI, there are cases where NPA
works for you while TE doesn't. The same thing goes for PI. PI might work for you while NPA/TE doesn't.
For
me, NPA is more effective than PI and TE. My guess is that NPA reacts better to my body chemistry.
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