Unless I'm sorely mistaken, PIA currently accepts Bitcoin. However, BTC is not as anonymous as it could be and, PIA being a service that focuses on providing greater privacy for its customers (in the form of a VPN), I think it's a good idea to also accept forms of payment that are themselves designed to provide more privacy to their users. In this case, Monero, Alias, and a few other coins.
Information regarding accepting Monero can be found here:
Information regarding accepting Alias can be found here:
My understanding is that it's a bit easier to start accepting Alias as a merchant than it is to start accepting Monero, but I could be mistaken. Regardless, I'd like to see PIA accept at least one of these coins, if not both, as forms of payment.