I agree that they should offer private and anonymous payment like monero and cash. They do talk about using a VPN and Tor to hide your IP but its kind of hidden in the footer.

https://proton.me/tor

https://proton.me/support/payment-options

Quote:

You can pay for any Proton subscription with Bitcoin — but it won’t appear as a payment option when you first create your account.

To use Bitcoin, create a free Proton Account, then upgrade to a paid subscription in your account settings.

Quote:

We accept cash payments in US dollars (USD), euros (EUR), or Swiss francs (CHF).