Buying a Mullvad scratch card is probably the most practical anonymous method. Usually the fact that you are using Mullvad at all isn't a secret (your ISPs can see you connecting), so outside of a very overcomplicated scenario where Amazon/$yourlocaltechstore are colluding with Mullvad to track individual scratch cards, it's fine.

Mailing cash in an anonymous envelope has a certain charm, but OTOH I have consistently had terrible experiences with the Swedish postal service and that seems to be a widespread opinion.