How have you set up your tables here and what kind of layout will you use?
If you want this format:
customer Name Average
John Smith 22.3
Fred Jones 21.2
and so forth, you can use a summary field for the average, but place the customer name field and summary field inside a sub summary part sorted by customer, then delete the body part so that only the names and averages are displayed.
This is just one approach. There are also other ways to do this.
That's what I did. Works nice!