On Mullvad's end, they also don't have to keep track of which gift card was used with which account, they just have to mark off that gift card as spent and cred

On Mullvad's end, they also don't have to keep track of which gift card was used with which account, they just have to mark off that gift card as spent and credit the account, unlike payment methods where they have to retain billing-to-account linkage.