I see the importance of adding a user that can only manage invoices but can't edit or delete customers in QuickBooks Online.
In QuickBooks, you can add different types of users. You have the option to add someone with limited access to the account. If they have access to customers and vendors, they can enter quotes, invoices, sales receipts, credit notes, refunds, charges, and credits. However, they can add, edit, and delete customers, suppliers, products, and services.
Since this feature is not available in QuickBooks Online, I suggest sending us a feature request.
Click the Gear icon, then select the Feedback button.