With the help of the report, you can see a summary of all sales per customer.
You can find the report in Solo under Contacts > Show report. You can view transactions for the desired time period and VAT-exclusive or VAT-inclusive.
You can also get a summary of the customer's invoices under Contacts > Show Contacts. Select the customer whose information you want to view, go to the Invoices tab and select Summary of selected invoices.