I usually ask clients to send over sensitive passwords and keys via iMsg if possible.
If that's not possible I use
It encrypts and gives you a cut and paste URL/pass to view, it also times out after a specified time.
and ask client to send user/password separately, and not to include any identifying information as to what service it's for.
Just have a form over HTTPS that store it somewhere in your Vault so that you can move it where appropriate later and add the instructions for power users that want to use PGP before sending it on the page.
It will be convenient and secure for everybody, and users wanting an extra level of security can use PGP.
A form on a webpage secured with HTTPS. If they have questions about security, answer them and add it to an FAQ. Anything with a middleman seems like just a bad idea.
may be worth looking into dedicated services like Yopass
1Password's guest mode is quite convenient and definitely provides a measure of confidence for the client. I use this with a number of clients and it works well for users of most technical levels.
Once they transfer a secret to us, I just move it out of the guest vault into another one. When I transfer a secret to them, I ask them to confirm that they have it and then I remove it from the shared vault.