Given that an E2EE solution requires all online stores switching technologies, it's unlikely to happen. The next best option is using a VPN-like solution for email. I use Privacy Portal email aliases with email encryption for this. There are multiple other alternatives but I like Privacy Portal because it has one of the strictest privacy policies and because I'm a little biased (I'm an engineer on the team).
Emails sent to you from online stores get sent to Privacy Portal's relay servers. These servers act like VPN servers meaning no logs, no writing to disk, zero storage, ... The emails get encrypted in memory with your public PGP key (or certificate) and get sent encrypted to your email provider. Only you will be able to decrypt them on device.
If you use Proton mail as your email provider, it supports PGP encryption by default. You can simply copy your public PGP key from proton and submit it to Privacy Portal and you're done. Proton won't have access to your emails. Alternatively you can use any email provider with an email client that supports PGP (e.g. Thunderbird, K9). And if all else fails you can even use S/MIME with Apple Mail on iOS but that has some drawbacks.
With this solution you would be separating providers into 2 categories:
- The first provider receives the unencrypted data but has no authorization to log or store anything.
- The second provider is responsible for storing your encrypted emails but does not access the unencrypted version.
On top of that, you can also reply to emails without exposing your the unencrypted versions to your email provider because encrypted emails sent by Privacy Portal contain public keys used for decrypting outbound mail before relaying it to its destination.
The cherry on top is that the online store won't have access to your personal email. So if they start spamming you, you can stop the email alias.