You can actually pay Posteo in cash by mail: send banknotes in an envelope with a code to Posteo.
@Tutanota as alternative does not offer direct cash payment, but you can buy Tuta gift cards cash same way via the reseller Proxystore.
@protonprivacy also accepts cash payments according to their support, by sending physical money via post. You get the details by contacting Proton support…
But if you pay with a Creditcard, you have an US provider an board