Per the forum thread, a better workflow for billable tickets would be:

• Ticket is flagged as billable
• Quote & Invoice options show
  • You can still jump straight to invoicing if you want
• Ticket is quoted (new column in tickets for ticket_quote_id)
• Quote is drafted, sent & reviewed by client
• Quote accepted:
  • Note added to ticket, quote automatically turned into draft invoice & associated with the ticket
• Declined:
  • Ticket marked non-billable & note added to the ticket