When a service provider submits an invoice for their work, it will show up on the Work Order detail page under the Invoices tab.
You (or the people responsible for approving these invoices at your company) will also receive a notification that the invoice has been received and is pending approval.
The invoice can be approved by navigating to the Work Order detail page's Invoices tab and clicking Approve -- or on the Invoice detail page accessible from the Invoices page.
You can also dispute the invoice by clicking the Dispute button. The service provider will be notified that their invoice has not been approved and further communicate to resolve the dispute can be done via the Notes tab of the Work Order detail page or via normal phone and email communication channels.