Easy to use website, order online having read the T&Cs.
Receive a confirmation e-mail, send them the payments via your bank- quoting the correct reference number.
Receive confirmation of funds e-mail.
Cash delivered later by Royal Mail.
I would always be reluctant to trust Royal Mail or indeed any delivery service for short-notice delivery, and I would always give as much notice as possible when ordering less common currencies- like Omani rial etc.
Used properly, I think TravelFX is a superb service, with very competitive rates and fees.