This video explains how to merge Invoices.
Quickbooks Online Users: Quickbooks Online users do NOT have the ability to merge Invoices in Service Autopilot.
Quickbooks Desktop Users: We strongly recommend that you do NOT merge Invoices in Service Autopilot if you sync with Quickbooks Desktop. If you sync with Quickbooks Desktop, you should be aware that using the merge feature for Invoices will cause balance discrepancies in both Quickbooks and Service Autopilot because Quickbooks cannot merge Invoices .