The gift card is a scratch off and has a number that is used to fund your Mullvad balance. So Amazon doesn't know which instance of the gift card you ordered, meaning there's no link to your specific Mullvad account payment.
The authorities might know you ordered a gift card, but not which Mullvad account you funded it with.
The authorities might know you ordered a gift card, but not which Mullvad account you funded it with.