1.Your
AddToCart
class (from your code behind) is not visible in your ASP page because is part of a namespace that is different then the application default one.
2.The solution is to specify also the class namespace (
PayPalDemo.AddToCart
) like bellow:
<%@ Page Language="C#" AutoEventWireup="true" Inherits="PayPalDemo.AddToCart" Codebehind="AddToCart.aspx.cs" %>