right I just checked they do gift cards now.

I think I was mistaken, posteo accepts cash payment: https://posteo.de/en/payment

for most people

yes living in this dystopia one must say that most people do not have a specific threat model. but i get your point.

you could pay tuta in cash AFAIK. this makes it basically impossible to recover a trail from the mail. just FYI. (neglecting forensic madness on fingerprint analysis of the bills and letter you handed in.)