While it's possible to do as you indicate, there are complications when it comes to working with UI order data during / after posting.  Largely, there is the issue of Auto Clear (resets the order screen after posting).

Two approaches I'd consider for addressing this requirement are:
1. Change the Qty Ordered - or Qty Backordered on change of OEORDD / QTY SHIPPED
2. If you must run through all lines of the posted order after posting, I'd suggest a sub / function where you create and compose your data sources (record a macro changing one line of an order to zero out the qty ordered).  The order number can be passed from the UI OEORDH data and run regardless of the state of the UI record 
2a. It's often easier (for me) to replace the post icon with my own, hide the default icon and then run my code BEFORE calling the post function 

In the OnRecordChanged event you're looking for these eReasons:

If (eReason = RSN_ADDNEW) Or (eReason = RSN_POST) Or (eReason = RSN_UPDATE) Then

You need to check out Orchid Extender, it's very powerful.  At Acumen, we don't do OE1110 custom screens with VB6 anymore, Extender scripts can do all the work.

Good helps:

Hide the button, make your own - totally never thought of it .  The normal eReason capturing does not work out well - I did try it.

very interesting comment Jay about using Extender - especially since Sage is trying to get rid of Macros.

Thanks all

I'm a ditto on adding your own Post button, I did that 100% of the time.  Before Extender...