> you can buy Mullvad credits on Amazon

That is not the same thing.

You buy a Mullvad scratch card on Amazon and you redeem the token string.

Mullvad also offer the option to put your card number into the Mullvad website, and I'm sure many privacy conscious people would be very reluctant to do that.

Card payments these days leave far too big a trail. The bank knows, the intermediary (e.g. Stripe) knows, and the merchant (e.g. Mullvad) has to keep records for accounting/tax requirements.