

Sorry to reply to a two week old comment, this is their supported payments page: https://proton.me/support/payment-options.
You can use a prepaid visa to buy account credits, or proton gift cards. They don’t support monero, which is really annoying, but it’s not the biggest hassle to convert from monero to bitcoin for a purchase.
YSK, it’s not open source. It’s not even source available for that matter. They’ve promised to go open source for a while now, but not much progress has been made.
https://browser.kagi.com/faq.html#oss