You can already pay for proton mail with btc. I always do.
It is not an option for me here in Ireland. https://image.nostr.build/edfee72943320747f07e84f317f879a99362b29ae4ae40935fc82a5d68eadc7f.jpg
I dont think you do it in quite the same way. You add credit then convert that into the subscription.
Try this https://proton.me/support/pay-with-bitcoin
I will try right away. Thanks for the info.