How to integrate PayPal in .net applications?

protected void Button1_Click(object sender, EventArgs e)
{
            string redirecturl = "";
            //Mention URL to redirect content to paypal site for testing i send to sandbox account
            redirecturl += "https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&business=" + ConfigurationManager.AppSettings["paypalemail"].ToString();    //mention here your paypal business id        

            //First name i assign static based on login details assign this value
            redirecturl += "&first_name=ravindran";

            //City i assign static based on login user detail you change this value
            redirecturl += "&city=trichy";

            //State i assign static based on login user detail you change this value
            redirecturl += "&state=tamilnadu";

            //Product Name
            redirecturl += "&item_name=" + "Horlicks"; 

            //Product Amount in dollor
            redirecturl += "&amount=" + "4";           
           
            //Shipping charges if any
            redirecturl += "&shipping=5";

            //Handling charges if any
            redirecturl += "&handling=5"; 

            //Tax amount if any
            redirecturl += "&tax=5";
            
            //Add quatity i added one only statically 
            redirecturl += "&quantity=1";

            //Currency code 
            redirecturl += "¤cy=USD";

            //Success return page url
            redirecturl += "&return=" + ConfigurationManager.AppSettings["SuccessURL"].ToString();

            //Failed return page url
            redirecturl += "&cancel_return=" + ConfigurationManager.AppSettings["FailedURL"].ToString();

            Response.Redirect(redirecturl);
}