I'd treet each service call as a separate Invoices record with your standard services listed in Products. That way a given invoice can list multiple services (for more than one Piano if necessary ) in the LineItems table. Is that what you have set up?
If so, you can place a portal to Invoices on your customer page to list all the service calls for that customer. If, instead, you want to list the individual services performed for that customer, we'll need to set up another relationship so that you can list Line Item Records for that customer--not something that would be hard to do here.