Alex have you done a trace in Epicor on how an order is created?
You should be calling the order creation process approximately in this fashion
GetNewOrderHed
ChangeCustomerInfo
Update
GetNewOrderDtl
SetLineDetail
Update
You can’t (at least not reliably) send the header and lines at the same time because of dependency issues, header needs to exist before the lines are added.
Do a trace in Epicor and then use my Trace tool (or Notepad) to look at the trance and see what data needs to be changed, send on every one of the calls.