Subtotal + index column

You can try this:

let
    Source = Excel.CurrentWorkbook(){[Name="Table2"]}[Content],

    // Group by Invoice No and Invoice Date to get sum of amount incl taxes
    Invoices = Table.Group(Source, {"Invoice No", "Invoice Date"}, {{"Amount Incl. Taxe", each List.Sum([Amount Incl. Taxe])}}),

    // Combine the sub-total rows with the original data
    Stacked = Table.Combine({Source, Invoices}),

    // Sort so sub-totals are at top of each invoice date
    Sorted = Table.Sort(Stacked, {{"Invoice Date", Order.Ascending}, {"Reference", Order.Ascending}}),

    // Add index to each invoice no
    Indexed = Table.Group(Sorted, {"Invoice No"}, {{"Rows",each Table.AddIndexColumn(_, "Index", 1)}}),

    // Remove extra column
    RemoveInvoiceNo = Table.RemoveColumns(Indexed,{"Invoice No"}),

    // Expand groups with Index as first column
    Result = Table.ExpandTableColumn(RemoveInvoiceNo, "Rows", {"Index"} & Table.ColumnNames(Source))
in
    Result