If you want to buy a Protonmail Subscription using BCH, here is how:

shop.protonmail.com

Go to shop.protonmail.com, scroll down and click on one of the available gift-cards you can print at home.

Continue to checkout as usual, fill in your Details and then continue to the Payment selection. You will be presented with this screen:

As you can see, we can pay using coinbase Commerce, so select coinbase Commerce and continue.

The following images speak for themselves:

You will then receive two mails, click on the one from "SendOwl".

The mail will look like this:

Copy the "upgrade code" and go the "Use giftcode" as shown below.

Paste in the previously copied code and click on "APPLY".

And you will have successfully added balance to your Protonmail-Account using Bitcoin Cash!

Additional Note: It seems like my previous Euro-Balance was 1:1 converted to USD, i reported this as a bug. I will update here once i hear from them. But this shouldn't be a problem for someone with no balance to start with or new users.

EDIT: I got the following reply from them:

Please note that when you make a currency change on your ProtonMail account, and if you already had credits on the account, a currency conversion does not take place.
Instead, the number of available credits remains the same, but the currency changes.
The reason for this is, ProtonMail's prices are the same (in regards to numbers) regardless of the currency you have chosen.
For example, the annual base ProtonMail plan costs EUR/USD/CHF 48.
The number is the same regardless of the currency, therefore, even if the credits are now stated in USD, that does not mean the credits are worth less than before.