BINGO! 
D) PROTON’S ANONYMOUS PAYMENT OPTIONS VIA RESELLERS ARE EXTREMELY LIMITED.
Tuta. Addy. SMS Pool. SimpleLogin. They all allow their gift cards to be purchased anonymously via resellers like the Proxy Store, which accepts cash and Monero.
Not Proton.
The Proxy Store is a popular digital store in the privacy community that is supported by many privacy companies.
a) Proton doesn’t sell their gift cards in the Proxy Store.
Proton sells subscription vouchers that can only be used to buy a subscription and nothing else. You cannot credit your account with them like you would with Tuta, Addy, SMSPool, or even SimpleLogin. This is a huge limitation
b) Proton’s Proxy Store vouchers are exclusive to new users.
An existing Proton subscriber cannot use them to renew their subscription. If they want to use them, they have to cancel their current subscription, which means losing any discount if you’re on a lifetime discount.
Even if you are a new user and you purchase a subscription anonymously via the Proxy Store, you cannot renew it via the Proxy Store. You will always have to cancel your current subscription or wait for it to expire in order to purchase another one the same way. That is not practical at all.
Any existing Proton subscriber should be able to renew their current subscription anonymously and automatically. And any new subscriber should be able to do the same.
c) Proton vouchers in the Proxy Store are not widely available
Some services that Proton supported (Proton Mail) are not supported anymore, and it seems that Proton is deliberately withholding those vouchers from the Proxy Store. I know this because I spoke directly to both Proton and the Proxy store about it.
Via their competition, Proton has plenty of great examples of services that support anonymous payments directly and via resellers. And yet, Proton refuses to follow that model. They refuse to follow the model of SimpleLogin, their own company. None of their direct payments are anonymous, and their indirect payment methods are deliberately limited so that only first time users can benefit from them, and only for the first year.
PROTON DOESN’T WANT TO SUPPORT ANONYMOUS PAYMENTS
It’s the only conclusion one can come to when you look at their deliberately poor implementation. They need to do better because they are outshined by their competition in this department.
If Proton truly supported anonymous payments, they would allow direct cash payments for gift cards and also sell those gift cards via privacy friendly resellers like the Proxy Store. They don’t. In addition, they would not require you declare your username for cash payments.
If Proton truly supported anonymous payments and were completely transparent about their data retention policy, many of their public controversies would have been avoided.