I always used Fedex. We would use the envelope they provided and stuff it full. I dont know how much you have, document wise. I know when I last had hubby send me documents from the UK, it was 37.00 pounds for overnight service..which equaled to about 65.00 at the time. For the reliability, and speed, you cant beat it. I know the UK postal service is much higher, as is ours. They were going to charge him over 45.00 pounds for the same thing, and it would have been here in 3-4 days..not overnight.
And..since my husband is not good at these things. I called FEDEX UK and scheduled the pick up myself, and paid with my debit card. They brought him the envelope, and shipping label, all he had to do was fill in the address portion.