According to this article (in german,
http://www.golem.de/news/protonmail-paypal-blockiert-sicheres-e-mail-projekt-von-cern-und-mit-1407-107582.html) bitcoin is a payment option, however I haven't checked it personally.
Onkel Paul
Update: there's a paragraph about bitcoin on their
indiegogo page:
Do you have a BitCoin (BTC) address I can donate to?Yes, you can get our BTC details
here. If you want a perk, email us your details at indiegogo@protonmail.ch!