true instead of True (C#)

As a boolean (bool), the values will always be "True" or "False". If you want to represent these differently when converted to a string, you can do the following in your view:

@Model.IsAdded.ToString().ToLower()

This will not work as IsAdded is boolean and "true" is string

IsAdded = (sessionStore.CheckExistanceOnSummary(product.productId) ? 
    "true" : 
    "false")

Try using it in javascript by converting it toLowercase().