Unless it's a small flat rate box, doesn't the customs number automatically track for usps priority mail international? I pretty much always ship USPS priority myself. it's terrible tracking. basically shows item was picked up, and at some point in the distant future might show it was delivered. to get slightly better tracking you need to send it registered mail, but like i said, priority + registered is almost the same as express and sometimes can be more expensive (just sent something to japan where this was the case). and this way it's as good as ups/fedex tracking because it's updated at every point along the way, and many countries have agreements with usps so you get guaranteed delivery dates or refund of your postage cost.