Hello,
Could you please put break point in this BADI and then execute the Shopping cart push through Function module BBP_PD_SC_TRANSFER and see if you are seeing value of l_wa_item-zcolle correctly populating cs_po1_document-is_poheader-collect_no or not?
Also, i am not sure you will need these two statements;
cs_po1_document-is_poheaderx-collect_no = abap_true .
cs_po1_document-is_poheaderx-our_ref = abap_true.
Please try to comment and also process the fields.
Thank you
Ritesh