This could easily be the start of another post, but here goes nothing.
TL;DR: SimpleLogin allows anonymous payments, Proton does NOT.
SL GIFT CARDS CAN BE PURCHASED ANONYMOUSLY
SimpleLogin gift cards can be purchased anonymously from the Proxy Store, which is a trusted third party merchant in the privacy community. I have successfully bought different products from them before. They accept cash and Monero which is the only anonymous cryptocurrency.
The Proxy Store accepts anonymous payments
The Proxy Store’s protocol for accepting cash is truly anonymous. Firstly, you are not directly paying the company whose product you are purchasing (Simple Login). SimpleLogin will never be able to trace who purchased the gift card.
The Proxy Store doesn’t require you revealing your identity
Secondly, the Proxy Store will also never know the identity of who purchased from them. The name on the envelope can easily be faked, and the content of the letter only needs to contain the cash and a 6 digit alphabet code (ABCDEF). Nothing else. If the post office or a bad actor opened the letter, they would have no idea what it means nor what it is for.
Thirdly, once the Proxy store receives your letter, they optionally email you with a message simply saying that your purchase is complete and that your gift card can be claimed. That is it. Nothing else. You then claim your gift card with a link that you must have saved and kept when you generated your purchase code, before sending the letter.
Proxy Store gift cards can be applied to any account
Your gift card can then be applied to any SimpleLogin account. It doesn’t have to be yours. You can gift it to a friend, a family member, or to a complete stranger on the internet anonymously if you wish to. All you have to do is share the link with that person.
PROTON DOES NOT ACCEPT ANONYMOUS PAYMENTS
Cash payments are merely pseudonymous
Proton accepts cash payments, but they require that you declare your Proton username, which compromises your privacy. You have to write it in the letter, meaning that there is a real person at Proton that will upgrade your account internally and see your profile.
For many Proton Mail users, if not the overwhelming majority, your Proton username is linked to your real identity. It’s probably your real name. Even if you used any of your secondary pseudonymous Proton Mail addresses, they are still linked with the one with your real name, which reveals your identity.
Moreover, you can’t make a Proton cash payment for a third party (friend, family, stranger) without them sharing their username with you and compromising their privacy. Proton’s name also has to appear on the envelope, unlike the Proxy Store, which only requires their address.
Proton gift cards cannot be purchased with cash
Proton has gift cards, which can only be bought directly through them. They are not sold by any third party merchants like the Proxy Store. In addition, they can only be purchased via KYC methods, i.e. credit card, PayPal or cryptocurrencies that are not anonymous (not Monero). They cannot be purchased with cash. That means that they are traceable to you and/or whoever you gift them to, and hence they compromise your privacy.
It’s really ironic that on the one hand Proton cash payments require you declare your username, and on the other, gift cards, which don’t require it, can’t be purchased with cash. It makes you wonder if it’s intentional.
CONCLUSION: PROTON PASS & SIMPLE LOGIN ARE NOT 100% FULLY MERGED
Because SimpleLogin and Proton Pass have separate subscriptions, and separate payment processing companies, it is not possible to apply an anonymously purchased SimpleLogin gift card to any native Proton subscription, nor is it possible to apply a Proton gift card to an SL subscription.
Even beyond anonymous payment methods, it is not possible to pay for an ongoing Proton Pass subscription via Simple Login or vice versa via any payment method, because the subscriptions are separate.