Mandatory check while submitting request in web portal


We have a requirement to have a Mandatory check box which end users need to select while submitting request in web portal. We need to enable this only for certain service items.

We created a custom attribute(boolean datatype) in ShoppingCartItem  Table and in the request properties we have set it as mandatory property. However this is allowing the users to submit request even with out checking the attribute as we have defined the type as boolean.

Can some one please let us know how we can  throw an error if user is not checking the attribute ?