collect data on form and insert it to database

Hi,
 
I have wpf form and should collect data from two comboboxes (filled from two db tables - articles and services) and insert it to third table in database (called packages). The problem I don't know how to solve is that the number of elements can vary from insert to insert. Sometimes a package can consist of one article, sometimes of one article and two services, sometimes of three articles and one service and so on. I think the best way to do it is using datagrid but don't know how to populate two comboboxes inside of datagrid with database data from two different tables (articles and services).
Note that the field article is null in row where the field service is not null and vice versa.
 
Please let me know what is the way to solve this.
 
Thanks.