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.
It all comes down to trust in the end, but over time I've come to trust Mullvad more and more. One particular example that sticks out to me is that they ended subscription based billing, specifically because it required them to hold customer information that they didn't want to have.
I don't see anything in their terms of service / privacy policy re Leta. It'd be nice to know if they retain any sort of data at all (there's prominent mention to caching stuff, but just what are they caching?), regardless of whether it is tied to PII.
Some key points:
- Acts as a Google proxy, removes tracking links and caches results
- Only available for Mullvad paid users
- 100 free direct searches a day, unlimited cached searches (further search result pages count towards limit)
- Results cached over all users for 30 days