Email is a store-and-forward, asynchronous transport. You can use it to transport:
- plain text (the body of an email)
- styled text (the html body of a MIME email)
- attached files
But none of those options will work well in your use case. You'd have to try to parse the body of the email if you were looking in the body for update information. -- Bad idea
And if you're using email to transport ordered data such as csv or Excel files, uploading the files directly to the website would enable a much better user interface.
You shouldn't use Excel files either since it will be too complicated for your user--he'd need to get the column (field) names exactly right, the content cells would also need to be exactly right, etc. Eg I use an excel file for input on one of my systems and a constant problem is zip/postal codes since they look like numbers but can have significant leading zeroes. Few users know how to enter them...
Recommendation create a set of web forms in the administrative section of the website. Your customer will be happy and you'll be even happier due to lower volume of support calls.