For thoroughness' sake, here are all of Proton's payment methods.
They accept:
- Debit/credit
- PayPal
- Google Pay
- Bitcoin
- Cash (mailed)
- Bank transfer
If I were making an email address for a group allegedly connected to "arson, vandalism and doxing", I would've definitely gone with Bitcoin. The fact that all PM were able to do in the face of Swiss authorities acting at the behest of the FBI was connect the email address to the payment source – and that this one piece of information was easily preventable by the activist – is an endorsement to me.