how to insert multiple records in a table using other table key

on save button click this code i have written


int lastid;
SqlConnection con = new SqlConnection(strConnString);
SqlCommand cmd = new SqlCommand(strConnString,con);

cmd.CommandText = "sp_insertinvoice";
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Clear();


//cmd.Parameters
cmd.Parameters.AddWithValue("@invoice_date", txtDate.Text);
cmd.Parameters.AddWithValue("@Firm_name", txt_frmname.Text.Trim());
cmd.Parameters.AddWithValue("@Regis_id", txt_regno.Text.Trim());
cmd.Parameters.Add("@idcategory", SqlDbType.Int).Direction = ParameterDirection.Output;
cmd.Parameters.AddWithValue("@productname", ((TextBox)gridviewproduct.Rows[0].FindControl("grd_txt_productname")).Text);
cmd.Parameters.AddWithValue("@description", ((TextBox)gridviewproduct.Rows[0].FindControl("grd_txt_description")).Text);
cmd.Parameters.AddWithValue("@qty", ((TextBox)gridviewproduct.Rows[0].FindControl("grd_txt_price")).Text);
cmd.Parameters.AddWithValue("@price", ((TextBox)gridviewproduct.Rows[0].FindControl("grd_txt_price")).Text);



con.Open();
cmd.ExecuteNonQuery();
lastid = Convert.ToInt32(cmd.Parameters["@idcategory"].Value.ToString());




and procedure that i have created is this

ALTER PROCEDURE [dbo].[sp_insertinvoice] (
@invoice_date date,
@firm_name varchar,
@Regis_id varchar,
@idcategory int output,
@productname varchar,
@description varchar,
@qty int,
@price int
)
AS
BEGIN
SET NOCOUNT ON;
INSERT INTO tblInvoiceDetails(invoice_date,Firm_Name,Resigtration_no) VALUES
(@invoice_date,@firm_name,@Regis_id)
SET @idcategory=SCOPE_IDENTITY()
insert into tbl_ProductDetails values(@productname,@description,@qty,@price,@idcategory)
RETURN @idcategory
END

can you pls tell me where i am doing mistake because this code only insert one value at a time

please suggest me to right way to do this
i want to create invoice receipt form