I was not satisfied with current answers, as sometimes you need to check multiple products. If you do the same search for each product, it's really a waste, so I put it into a dispatcher format.
add_action('woocommerce_order_status_completed', 'onItemCheckout',10,1);
function onItemCheckout($order_id){
$order = wc_get_order($order_id);
foreach ($order->get_items() as $item_key => $item_values){
$product_id = $item_values->get_product_id();
switch($item_values->get_product_id()){
case 9999 : FreeShipping($order, $product_id); break;
case 1010 : RequireValidation($order, $product_id); break;
default: break;
}
}
}
Alternatively,...
$ItemCheckoutHandler=[];
$ItemCheckoutHandler[9999]='FreeShipping';
$ItemCheckoutHandler[1010]='RequireValidation';
add_action('woocommerce_order_status_completed', 'onItemCheckout',10,1);
function onItemCheckout($order_id){
global $ItemCheckoutHandler;
$order = wc_get_order($order_id);
foreach ($order->get_items() as $item_key => $item_values){
$product_id=$item_values->get_product_id();
$ItemCheckoutHandler[ $product_id ]( $order, $product_id );
} //Call the function assigned to that product id in the array
}
In either case, the assigned functions would take the order
object, rather than the id, and the product_id
as an argument:
function FreeShipping($order, $product_id){ ... }
function RequireValidation($order, $product_id){ ... }
You can of course customize these inputs to your liking.