UK postal orders can only be cashed in certain countries, which are mainly Commonwealth nations. They can't be cashed in the USA. (Post Offices used to offer International Money Orders, which were similar to Postal Orders but could be cashed worldwide. However they've now been withdrawn in favour of Moneygram - see below).
Western Union and MoneyGram (which is often easier to find, since it's available at all Post Office branches) offer useful services but they're far from cheap.
I've used Ikobo plenty of times, with no problems:
http://www.ikobo.com/
However, as Sara3 suggests, Paypal is probably cheapest:
https://www.paypal-ma....uk/sendmoney/how.htm
Chris