This action allows you to get four pieces of data in order to do something different with it after the reserve email has been sent.  This does not modify the info sent to the user, it is just available if you want to add it to another table in your database, send it to another person (like an admin of your site), etc.

function my_custom_function($inventory_id, $data, $subject, $message) {
$inventory_id; // Id of the inventory item that was just reserved
$data; // The array of information from the reserve form (not the cart data)
$subject; // Subject of the email
$message; // Message of the email

// Get the item information

$item_info = new WPIMItem(); // Get new instance of the item class
$item_info = $item_info->get( $inventory_id ); // use the get method with the inventory id to get item information

$name = $item_info->inventory_name;  // Example to fetch the item name

// Do whatever you want from here!

}

add_action('wpim_reserve_sent', 'my_custom_function', 10, 4);