Why not anonymous payment alternative(s)?
as it is seen among many ideas mentioned above, I think this is one of major missing features at present. For this sole reason I have hesitated to upgrade (specially VPN service). Until anonymous payment methods are provided, I will keep going with free account.
Bitcoin isn't anonymous. It's pretty easily tracked. Monero is a completely anonymous cryptocurrency, and you can make bitcoin payment with it using the xmr.to website. That's how I plan to pay for my ProtonMail upgrade. It'd be cool if they just accepted Monero directly, but I'm completely okay with the current state of things.
Paying with BitCoin: https://protonmail.com/support/knowledge-base/paying-with-bitcoin/