get a domain name, host stalwart somewhere and set up email with this new domain there, get receipt emails there and autoforward it to your main email with S/MIME, gpg or whatever enabled.
usual disclaimer ‘do not host your email blablabla’ (at least don’t get fucking digitalocean ‘droplet’ for it), but there’s no other way around that, ecommerce won’t enable shit.
Encryption in transit is pretty much solved these days with TLS, what OP wants is E2E - encryption from sender to recipient with no intermediate parties having an idea about contents of the message. Problem with E2E is inconvenience: emails are inaccessible without private keys and key management is pain. Users don’t want additional headache of managing their keys between bajillion of devices where they might use emails