Drupal Commerce Hook - Extra validation before adding to cart I was looking for a hook that will fire after a user hits the add to cart button.
The commerce api isn't very clear about this.

What I'm trying to is:
Add an extra validation for a customized product (date field). 
So basically I want to interrupt the adding to the cart process and proceed if validation is ok, and lead the user back if not.

Thanks!
Seems like hook_commerce_cart_product_add is what you're after. I couldn't find any examples for preventing the line item being added to the cart but at the very least you should be able to use drupal_goto to interupt the process. You might also want to look at the Rules events\actions provided by Commerce.