Implementing a paypal with a custom built shopping cart

Hi

I have developed my ecommerce website in asp.net which has its own custom shopping cart. Having read quite a bit about web payments standard it appears that you submit data using a form tag for processing your data at the paypal end.

I found an example of this process online which incidently does not work but there is enough code to demonstrate the process.

My Question is this.
If a user has filled up my shopping cart with say 5 items and different quantities how would I send this to paypal? If I bundle the items up under one title and send this as one transaction how will I know what items my customer has bought? My problem is, without capturing customer details I cannot store orders so how does this work?

On checkout.aspx.....
protected void btnPaypalCheckout_Click(object sender, EventArgs e)
    {
        Page.Validate();
        if (Page.IsValid)
        {
            //store data
            SelectedInvoice.ContactName = this.txtBillingName.Text;
            SelectedInvoice.ContactPhone = this.txtBillingPhone.Text;
            SelectedInvoice.ContactEmail = this.txtBillingEmail.Text;

            SelectedInvoice.ContactAddress1 = this.txtBillingAddress1.Text;
            SelectedInvoice.ContactAddress2 = this.txtBillingAddress2.Text;
            SelectedInvoice.ContactCity = this.txtBillingCity.Text;
            SelectedInvoice.ContactStateProvince = this.cboBillingState.SelectedValue;
            SelectedInvoice.ContactZip = this.txtBillingZip.Text;

            SelectedInvoice.ShipToAddress1 = this.txtShipToAddress1.Text;
            SelectedInvoice.ShipToAddress2 = this.txtShipToAddress2.Text;
            SelectedInvoice.ShipToCity = this.txtShipToCity.Text;
            SelectedInvoice.ShipToStateProvince = this.cboShipToState.SelectedValue;
            SelectedInvoice.ShipToZip = this.txtShipToZip.Text;

            SelectedInvoice.CustomerComments = this.txtCustomerComments.Text;

            Session["Invoice"] = SelectedInvoice;

            //redirect
            Response.Redirect("PayPalProcessing.aspx");
        }
    }

On PaypalProcessing.aspx....

Code Behind
private Invoice _selectedInvoice = new Invoice();
    public Invoice SelectedInvoice
    {
        get
        {
            if (_selectedInvoice.InvoiceId == "")
            {
                try
                {
                    _selectedInvoice = (Invoice)Session["Invoice"];
                }
                catch { }               
            }
            return _selectedInvoice;
        }
    }

HTML Section
<!-- item_number should get passed back -->
		<form id="Paypal" name="Paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
			
			<input type="hidden" name="cmd" value="_cart" />
			<input type="hidden" name="upload" value="1" />
			
			<!-- The following is for aggregated PayPal data instead of itemized -->
			<!--
			<input type="hidden" name="item_name" value="Aggregated Items" />
			<input type="hidden" name="amount" value="<%=SelectedInvoice.Total.ToString("#.00") %>" />
			-->
			
			<!-- The following is for itemized PayPal data instead of the aggregated version -->
			<%=SelectedInvoice.PaypalItemList%>
			<input type="hidden" name="tax_cart" value="<%=SelectedInvoice.Taxes.ToString("#.00")%>" />
			
			<!-- STANDARD DATA -->
			<input type="hidden" name="business" value="<%=System.Web.Configuration.WebConfigurationManager.AppSettings["paypalAccount"] %>" />
									
			<input type="hidden" name="invoice" value="<%=SelectedInvoice.InvoiceId.ToString()%>" />
			<input type="hidden" name="no_note" value="0" /> 
			<input type="hidden" name="currency_code" value="<%=SelectedInvoice.Currency %>" />
			<input type="hidden" name="lc" value="<%=SelectedInvoice.ShipToCountry %>" /> 
			<input type="hidden" name="return" value="<%=System.Web.Configuration.WebConfigurationManager.AppSettings["websiteUrl"] %>/ThankYou.aspx" />
			<input type="hidden" name="cancel_return" value="<%=System.Web.Configuration.WebConfigurationManager.AppSettings["websiteUrl"] %>" />
			<input type="hidden" name="email" value="<%=System.Web.Configuration.WebConfigurationManager.AppSettings["paypalAccount"] %>" /> 
			<input type="hidden" name="cn" value="How did you hear about us?" />
           <input type="submit" value="Submit Payment Info" />
		</form>
		<%=SelectedInvoice.ToString() %>

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