It is almost impossible to guarantee that email is secure.
Although you would need to be extremely careful doing this it is certainly more secure than email. With all other hacking vectors addressed and 2-way hashing the SS and storing it would be the better way to go.
- Store 2 way hashed SS
- Send a email with a link to their information.
- User clicks on email link so you need to ensure that a security challenge is the next action that occurs after they navigate to the link in their browser. You use a message like "As this link contains your private information then please ensure that you don't view this in a public place or public wifi network. For your safety we ask you provide your username and password to view this information.
- Present user authentication challenge
- if the authentication passes then let them view the web page that reports their secure information while also all aspects including images secured under https://
I would not trust a service that does this for you as you don't want to use a public and known algorithm or process. If you are handling this type of information then you need to be very aware of the proper ways to secure it.
For the hashing part of it, you might want to start off with PHP mcrypt to handle your two way encryption of the SS.
Shiflett.org is a good place for you to read about PHP security.
Please continue to be diligent with this as you have seemed to begun on the right track by not just assuming that your SSL secure site, email and unencrypted storage are not safe.