Hey!
Glad to hear you're trying ITFlow.
Unfortunately not easily, at the moment.
We need to add a way to link invoices and ticketing, all the button does currently is add a note to the ticket and the details to the invoice. I can't find the post, but I've floated the idea somewhere of having a field in the database to link the invoice number to, which would then allow you to easily find tickets that have / don't have an invoice.