I run an internet business and ship for a living. One of the biggest problems I face is customs and delivery on the other end. Customs can delay ridiculously long during the holidays. Some countries use private couriers for USPS international if you insure, and things are sometimes stolen. I have had to learn which countries are notorious for theft, and the best way to ship to each of them.
Each country has their own agreement with shippers. Some allow the US business end (UPS, USPS, Fed Ex) to look 'into' their system to track packages; others don't. On my end, tracking ends with a message 'in customs' and then I have to hand the tracking number to the customer, who tracks it on their end through their postal services. Can be frustrating for both shipper and shippee.
I like shipping to the UK because I get definitives, right up to 'it's been delivered'. I once shipped a diamond and platinum watch to a customer in the UK for his wife for Christmas, and I KNEW it was waiting for him at the post office, but when he went in, they kept saying it wasn't there. Christmas eve I told him to go into that post office and go all "New York" on them - bang is fist on the counter and demand it. He called a 1/2 hour later to tell me he had the package. Sometimes ya just have to go all "Ugly American"