Hi,
Firstly not sure why do we need this program for?
Is there any specific Business requirement for this? Why can't we use standard output technique which will trigger it via standard output mechanism and there are loads of standard reports available for re-processing. So IMO you should challenge the requirement first.
Second, considering we have to do this.
I understand RLB_INVOICE populates the NAST entry as it is supposed to be called via Output technique. You can anyway populate lwa_nast manulaly .
If it does not work for '3' or '5' as the transmission medium, as user enters, you can anyway overwrite it to '1' ( If it is working for '1' ) or whatever value it is working as you are writing a custom program. Can't you?
R