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.

Giftcards from Amazon will be enough of a stumbling block to stop copyright trolls and such.

it won't even slow down actual criminal investigations by nation states and might not even stop a determined civil suit.

Or send cash in an envelope.

Doesn't cost extra

No need to share my cc with yet another company.