Use the Complete Item Tasks action to complete any and/or all OPEN tasks on a Podio Item
Select the Flow Trigger and add any required Filters
Add the Get Item Task(s) action.
Set up a For Each loop and a Sanity Check to ensure ony specific tasks get completed.
Add the Complete Item Tasks action to your Flow to complete the OPEN tasks.
When the Status of a Work Order is changed to Invoiced, Get Items Tasks, find the one where the Title contains "Invoice" and Complete it. This will ensure that the task created to send out the Invoice is closed and removed from the Assignee's list.