Just to be clear, this is the code as it appears in my theme functions file:
/**
* Checks the cart to verify whether or not a product from a Category is in the cart
* @param $category Accepts the Product Category Name, ID, Slug or array of them
* @return bool
*/
function qualifies_basedon_product_category( $category ) {
foreach( WC()->cart->cart_contents as $key => $product_in_cart ) {
if( has_term( $category, 'fee', get_id_from_product( $product_in_cart, false ) ) ) {
return true;
}
}
// Return false in case anything fails
return false;
}
/**
* Adds a specific product to the cart
* @param $product_id Product to be added to the cart
*/
function add_incentive_to_cart( $product_id ) {
// Check the cart for this product
$cart_id = WC()->cart->generate_cart_id( $product_id );
$prod_in_cart = WC()->cart->find_product_in_cart( $cart_id );
// Add the product only if it's not in the cart already
if( ! $prod_in_cart ) {
WC()->cart->add_to_cart( $product_id );
}
}
add_action( 'woocommerce_check_cart_items', 'qualifies_for_incentive' );
function qualifies_for_incentive() {
// Incentive product we are giving away
$incentive_product_id = 506;
if( qualifies_basedon_product_category( 'fee' ) ) {
add_incentive_to_cart( $incentive_product_id );
}
}