An invoice merge is an implicit merge print. The head or parent data contains one record for each document which is linked 1:n with the detail or child data. To design and to print such a document the parent table has to be passed to List & Label by the DataMember property. Furthermore the AutoMasterMode property has to be set to AsVariables as shown in the following example.

using (ListLabel LL = new ListLabel())
{
    // Define/Assign data source
    LL.DataSource = CreateDataSet();

    // Order data as variables
    LL.DataMember = "InvoiceHeader";
    LL.AutoMasterMode = LlAutoMasterMode.AsVariables;

    // Call the Designer
    LL.Design();

    // Print
    LL.Print();
}

Using LL As New ListLabel()
    ' Define/Assign data source
    LL.DataSource = CreateDataSet()

    ' Order data as variables
    LL.DataMember = "InvoiceHeader"
    LL.AutoMasterMode = LlAutoMasterMode.AsVariables

    ' Call the Designer
    LL.Design()

    ' Print
    LL.Print()
End Using