If you finalise a delivery you will be asked if you want to  confirm the finalise, and if you OK this, then you are asked if you want to invoice the supplier. Assume you say OK.

The delivery now has status 'Posted', and there is a invoice to the supplier.

However, on the deliveries screen the Invoice button is available for a Posted delivery.

I suspect that the problem is that the problem is that we really need the Posted status to be split in two, Posted (meaning finalised delivery - no more edits allowed), and then when the invoice is created 'Billed' (or Invoiced).

Then you could display the Invoice button if the status was posted/finalised but not if it was Invoiced.

Regards, Tim G