Realistically, you might have to send cash. Even that doesn't work out particularly cheaply. First of all, you can't get $17.70 from the Post Office (or other bureau de change). You'd have to get $20. (You'd also need to watch out for fees. Some bureaux de change charge a �6 minimum fee, plus commission to change your money). Then you'd have to pay at least 54p postage. On top of that, you'd need to consider using the 'AirSure' service, which costs �4.20 extra. ('AirSure' mail is electronically tracked).
Services like Western Union and MoneyGram are far too expensive. (The minimum fee with MoneyGram, which is available through any post office, is �12).
Your only other option might be to persuade the seller that, if they want to sell online and not use PayPal, they should consider accepting payment by Ikobo. If they do, you pay $17.70, plus a fee of $5.51, online to Ikobo. They then send the seller an e-mail to state that you've paid. They also send the seller a Visa debit card, with $17.70 in the account. In future, the seller can just ask for payment by Ikobo. As soon as a payment is made, he gets an e-mail from Ikobo and the amount is credited to his debit card account:
http://www.ikobo.com/
Chris