Function to add product to cart programatically in drupal commerce.

add_product.module

<?php

function my_module_product_add($pid,$quantity=1){

    if ($product = commerce_product_load($pid) ) {
            global $user;
            //uid of the logged in user
            $uid = $user->uid;
            $line_item = commerce_product_line_item_new($product, $quantity);
            $line_item->field_ticket_type['und'][]['value'] = 0;
            $line_item = commerce_cart_product_add($uid, $line_item, FALSE);
            
            //optionally set message if you want
            drupal_set_message("Done! You just did it.");
        }
}