This will only work for PayPal, obviously, but several other payment
gateways have similar mechanisms:

private void BindData()
{
 Dictionary<string, byte> dicCart = (Dictionary<string,
byte>)Session["dicCart"];
 string strPayPalForm;
 DataRow objRow = null;
 LinkButton lnkRemove;
 decimal curItemTotal = 0;
 decimal curSubTotal = 0;
 decimal curShipping = 0;
 decimal curShippingTotal = 0;
 decimal curTotal = 0;

 try
 {
  if(dicCart.Count == 0)
  {
   tblCart.Visible = false;
               ClientScript.RegisterStartupScript(GetType(), "emptyCart",
"alert('There are no items in your shopping
cart');window.location='default.aspx';", true);
  }
  else
  {
   tblCart.Visible = true;

   strPayPalForm = String.Empty;

               strPayPalForm += "<form target=\"paypal\" id=\"frmPayPal\"
action=\"" +
System.Configuration.ConfigurationManager.AppSettings["PayPalURL"] + "\"
method=\"post\">\r\n";
   strPayPalForm += "\t<input type=\"hidden\" name=\"cmd\" value=\"_cart\"
/>\r\n";
   strPayPalForm += "\t<input type=\"hidden\" name=\"upload\" value=\"1\"
/>\r\n";
               strPayPalForm += "\t<input type=\"hidden\" name=\"business\"
value=\"" +
System.Configuration.ConfigurationManager.AppSettings["PayPalAccount"] + "\"
/>\r\n";
   strPayPalForm += "\t<input type=\"hidden\" name=\"currency_code\"
value=\"GBP\" />\r\n";

   using (DataSet objDS = new DataSet())
   {
    objDS.ReadXml(Request.PhysicalApplicationPath +
"\\App_Data\\merch.xml");
    DataColumn[] objPK = new DataColumn[1];
    objPK[0] = objDS.Tables[0].Columns["id"];
    objDS.Tables[0].PrimaryKey = objPK;

    int intItem = 1;

    foreach (KeyValuePair<string, byte> kvpItem in dicCart)
    {
     objRow = objDS.Tables[0].Rows.Find(kvpItem.Key);

     using (TableRow objTR = new TableRow()) // create a new row
     {
      // item -----------------------------------------------------
      using (TableCell objTD = new TableCell())
      {
       objTD.Text = objRow["item_type"].ToString();
       objTD.HorizontalAlign = HorizontalAlign.Left;
       objTR.Cells.Add(objTD);
      }

      // description ----------------------------------------------
      using (TableCell objTD = new TableCell())
      {
       objTD.Text = objRow["item_description"].ToString();
       objTD.HorizontalAlign = HorizontalAlign.Left;
       objTR.Cells.Add(objTD);
      }
      strPayPalForm += "<input type=\"hidden\" name=\"item_name_" +
intItem.ToString() + "\" value=\"" + objRow["item_description"].ToString() +
"\" />\r\n";

      // quantity -------------------------------------------------
      using (TableCell objTD = new TableCell())
      {
       objTD.Controls.Add(new LiteralControl(kvpItem.Value.ToString() +
"&nbsp;"));
       lnkRemove = (LinkButton)pnlCart.FindControl("lnkRemove_" +
kvpItem.Key);
       lnkRemove.Visible = true;
       if (kvpItem.Value == 1)
       {
        lnkRemove.ToolTip = "Remove this item from your shopping cart";
       }
       else
       {
        lnkRemove.ToolTip = "Reduce the quantity of this item";
       }
       objTD.Controls.Add(lnkRemove);
       objTD.HorizontalAlign = HorizontalAlign.Right;
       objTR.Cells.Add(objTD);
      }
      strPayPalForm += "<input type=\"hidden\" name=\"quantity_" +
intItem.ToString() + "\" value=\"" + kvpItem.Value.ToString() + "\" />\r\n";

      // unit cost ------------------------------------------------
      using (TableCell objTD = new TableCell())
      {
       objTD.Text = objRow["item_cost"].ToString();
       objTD.HorizontalAlign = HorizontalAlign.Right;
       objTR.Cells.Add(objTD);
      }
      strPayPalForm += "<input type=\"hidden\" name=\"amount_" +
intItem.ToString() + "\" value=\"" + objRow["item_cost"].ToString() + "\"
/>\r\n";

      // total cost -----------------------------------------------
      curItemTotal = Convert.ToDecimal(Convert.ToDecimal(kvpItem.Value) *
Convert.ToDecimal(objRow["item_cost"].ToString()));
      curSubTotal += curItemTotal;
      switch (cmbShipping.SelectedValue)
      {
       case "1":  // UK
       {
        curShipping = Convert.ToDecimal(objRow["item_ship_uk"].ToString());
        strPayPalForm += "<input type=\"hidden\" name=\"shipping_" +
intItem.ToString() + "\" value=\"" + curShipping.ToString() + "\" />\r\n";
        if (kvpItem.Value > 1)
        {
         strPayPalForm += "<input type=\"hidden\" name=\"shipping2_" +
intItem.ToString() + "\" value=\"" + curShipping.ToString() + "\" />\r\n";
        }
        curShippingTotal +=
Convert.ToDecimal(Convert.ToDecimal(kvpItem.Value) * curShipping);
        break;
       }
       case "2":  // EU
       {
        curShipping =
Convert.ToDecimal(objRow["item_ship_europe"].ToString());
        strPayPalForm += "<input type=\"hidden\" name=\"shipping_" +
intItem.ToString() + "\" value=\"" + curShipping.ToString() + "\" />\r\n";
        if (kvpItem.Value > 1)
        {
         strPayPalForm += "<input type=\"hidden\" name=\"shipping2_" +
intItem.ToString() + "\" value=\"" + curShipping.ToString() + "\" />\r\n";
        }
        curShippingTotal +=
Convert.ToDecimal(Convert.ToDecimal(kvpItem.Value) * curShipping);
        break;
       }
       case "3":  // rest of world
       {
        curShipping =
Convert.ToDecimal(objRow["item_ship_world"].ToString());
        strPayPalForm += "<input type=\"hidden\" name=\"shipping_" +
intItem.ToString() + "\" value=\"" + curShipping.ToString() + "\" />\r\n";
        if (kvpItem.Value > 1)
        {
         strPayPalForm += "<input type=\"hidden\" name=\"shipping2_" +
intItem.ToString() + "\" value=\"" + curShipping.ToString() + "\" />\r\n";
        }
        curShippingTotal +=
Convert.ToDecimal(Convert.ToDecimal(kvpItem.Value) * curShipping);
        break;
       }
      }
      using (TableCell objTD = new TableCell())
      {
       objTD.Text = curItemTotal.ToString("#,##0.00");
       objTD.HorizontalAlign = HorizontalAlign.Right;
       objTR.Cells.Add(objTD);
      }

      tblCart.Rows.AddAt(intItem, objTR); // add the row to the table
     }
     intItem++;
    }
   }

   lblShipping.Text = curShippingTotal.ToString("#,##0.00");
   curTotal = curSubTotal + curShippingTotal;
   lblTotal.Text = curTotal.ToString("#,##0.00");
   strPayPalForm += "<input type=\"button\" value=\"Continue shopping\"
onclick=\"location.href='default.aspx';\" />\r\n";
   strPayPalForm += "&nbsp;\r\n";
   strPayPalForm += "<input type=\"submit\" value=\"Proceed to checkout\"
onclick=\"return submitPayPal();\" />";

   strPayPalForm += "</form>";
   ((Literal)Master.FindControl("litAdditional")).Text =
strPayPalForm.Replace("\r\n", String.Empty);
  }
 }
 catch (Exception ex)
 {
  CApplication.GlobalExceptionHandler(ex);
 }
}