Validate Items in the cart before checkout

[Lithix]

I am integrating some ad removal features into WHMCS and I need to be able to validate some data when they add an item to the cart.

 

 

For example...when they click "ad removal" I have a custom drop down that populates with the domain names I serve. Then I have a text box where they can enter thier site name, which is thier subdomain. I need to validate this before they purchase, otherwise they are likely to enter something wrong or put in a site that doesn't exists, then I have to manually fix it.

 

Is there a way to validate the data on an item placed in the cart when they hit the "Add to Cart" button? I see the PreShoppingCartCheckout application hook but the documentation is seriously lacking on when this function is called and what data I have access to.

 

Can anyone help out as to how I can access those custom fields, and send an error message back?

 

Thanks,

JT