There seems to be quite a few questions about this same topic and from what I have found there is clear answer.

I have been looking into this Purchase workflow for sometime and can see there is a bit to do make it work into the existing workflow effectively and I do not believe it can be done from the database itself without making the end workflow less than perfect and may need a direction set by the core developer team.

One way I might suggest is to create a new state "PO Sent" that sits in between the "Purchase Order" and "Done" in the statusbar and for the workflow to match the "RFQ Sent". In the back end the purchase.py file will need to accomidate this.

In the email templates I can see some work has taken effect to make an automatic email to sent either a RFQ or PO depending on the status of the PO but from want I can see there may need to be a change to the core to allow "Optional report to print and attach" and "Report Filename" to accept a logical argument instead setting a single report.

Some newss?

you can try creating an RFQ for the same order with the price that you expect and then instead of sending the mail, click on confirm order straightaway, it gets added to incoming shipments. Please let me know if that works