You are really looking for a database backend with excel to drive the frontend. This requires some VBA scripting to query (SQL) the database and populate the listbox and simply open their last excel file, therefore your database needs to track filenames and you need to impliment a standard naming convention on the customer files so as they are unique yet predictable.
Therefore a number of questions arises
a) Do you have any such customer database in place?
b) Details of the database if any?
c) What is your budget?
I doubt there is any off the shelf solutions out there that will work exactly how you want and suggest developing this inhouse.