If you want to persist the changes you have to implement the
IF_EX_ME_PROCESS_PO_CUST->POST( )
method in your BAdI implementation.
You can use OPEN( ) if you want to prepare any header data before it is being edited through ME22N.
Sample (from a real project):
METHODif_ex_me_process_po_cust~open.
DATA:
mepoheader_raw TYPE mepoheader.
mepoheader_raw = im_header->get_data( ).
mepoheader_raw-inco1 = 'DDP'.
im_header->set_data( mepoheader_raw ).
ENDMETHOD.
Regards,
Chris