I live in the US and for UK autographs, I just go to royalmail.com. Click on "Get a price" at the top of the screen. Then answer the following questions:
1) Where are you sending to - click Overseas and specify USA
2) What are you sending - Choose whichever is appropriate to you and specify the weight and value. Note the weight is in grams, so adjust accordingly
3) What size is your parcel - only applicable if you're not sending a letter
4) How much will it cost - select your choice and click on "Buy Online". You can then buy the postage and print it out on your home computer.
Disclaimer - I've only done this twice. The one to Ian McKellen hasn't come back yet, and the one to Theresa May came back, but she used her own postage and didn't use my SASE.