To add the Invoices related list to the Opportunity Page, enter Setup in your Salesforce Instance and please follow the steps below:
Step 1 – Under Build navigate to Customize -> Opportunities and select Page Layouts.
Step 2 – Select Edit next to Opportunity Layout.
Step 3 – Select Related Lists and drag Invoices down to the Related Lists section.
Step 4 – Select Save to action the changes to the Opportunity Page.