They also have vouchers you can buy from Amazon, which I find a nice alternative to sending cash in an envelope.
On the other hand, if you don't trust Mullvad's assertion that they delete the link between accounts and credit card payment records after 40 days [1], what makes you think you can trust them to not keep a record of individual scratch cards sold on Amazon, which Amazon can then correlate to an order ID and by extension account and shipping address?
At a higher level, if somebody can convince Mullvad to collude in that manner, they can likely also just ask them to outright hand over your traffic flows and connection data.
By the end of the day I agree, if you have any "real" reason for using VPN you pretty much have to implicitly trust your provider to not keep any traffic flows and connections that could correlate traffic to your IP, but not even sending money in envelope saves your from that.
If your worried about anything in a 40 day window the credit card <-> account_id is a liability
Amazon doesn’t know the redemption code on the gift card. So Amazon knows that you purchased a Mullvad gift card, but can’t associate the transaction with a Mullvad account. Likewise Mullvad knows service was paid for with a gift card (possibly that the gift card is from a lot sold on Amazon). But they do not know which Amazon transaction the card is associated with.
Unless your behavior and the behavior of others deanonymizes the Amazon purchase <-> redemption your account should be indistinguishable from any other that purchased a Mullvad gift card from Amazon in that window of time.