When you are working on a job, then need to send an estimate. The parts on the estimate should be added to the ticket charges when the estimate is approved. Currently it only goes to a new invoice. Thats fine for when an estimate it created outside of a ticket, like quoting new PC or for some work.
But, when you have already done some billable time on a ticket or added parts and something outside of scope of the ticket comes up a new estimate is created from the ticket.
When the new estimate is approved, those items should be sent back to the ticket so the job can be invoiced as a whole.