It's almost impossible cost effectively to generate a small US dollar payment from the UK unless you have a dollar bank account. The "International Money order" is not obtainable in the UK (believe me I tried) and the equivalent from American Express or similar will double the cost of the payment. So will getting a US dollar bank draft or a bank transfer to a US bank.   

About 15 years ago the standard US bank deal for collecting a UK cheque was 28 days and a $28 charge or 7% of the total if greater.