Invoice Spliting for Foreign trade customers

Hi,

I'm doing 4 sales orders(ZPOR) and 4 Deliveries(ZLF) for Foreign Trade Customers

into One Invoice(ZPDS). Invoice is splitting the reason is giving split to different Header data, Number of Foreign trade is 5,6,7,8. The fogreign trade no is giving system internally delivery header foreign trade screen in organization. The system is giving 4 different foreign trade no in 4 delivereis.How can I do the One Invoice.