You can directly buy PIA access.
They accept Bitcoin, and charge 3.33$ per month if you buy a yearly subscription.
(If you use
this link sent to users whose subscription has just ended, you will get it for 20% off, for only 2.66$ per month if you order a year.
Also, they have a 7 day moneyback plan if you don't like it.
(Warning: Moneyback is not tied to BTC, but tied to USD. May be lesser if BTC rises in the 7 days)