What are you trying to edit with Orders? Just the total? Cause if it is just the total then Adjust Order Total button may suit your needs. This buttion may not be showing up for you however. We used the Adjust Order Total feature with credit cards before we were able to caclulate freight online.
Since you have a place to test then I would try doing few edits to orderframe.aspx.cs.
adding this adding this bool:
bool IsPO = (PM == AppLogic.ro_PMPurchaseOrder);
Then edit this if statment to have an adds an or to the if statment below
if ((IsCard || IsMicroPay || IsPayPal || IsCOD) && (GWSupportsAdHocOrders && GW != Gateway.ro_GWGOOGLECHECKOUT))
Does that do what you want?