In order to view transactional history such as Sales Orders, Invoices, Quotes etc, on Customer records you will need to add the Lists > Financial History permission to the users role.
Please view the following article on how to add a permission to a role: Managing Roles
Then when viewing a Customer record, goto Sales > Transactions subtab to see the Customers financial history.
Tip: Under the Transactions sublist, you can use the Type filter, to display specific transaction types, such as Invoice.
