In the days before internet banking, a postal order was an easy way to send money through the post to a recipient and did not require either party to have a bank account. It was secure as only the person named on the postal order could cash it and if you needed added security it could be crossed and then it could only be paid into a bank account or used to pay a bill at the post office.
Although postal orders are now mostly used for people to send money through the post in a secure way to relatives or friends they can be used to pay for goods online especially if an individual is purchasing items perhaps from an auction site and they do not want to give out their bank details. Not everyone has a PayPal account so sending a postal order is an easy way to pay for goods although it is important to keep in mind that there is no return guarantee as there is with an online payment system.
There is a small charge for a postal order starting at fifty pence for an amount less than five pounds rising to a maximum charge of twelve pounds fifty pence for postal orders valued at between one hundred pounds and up to the maximum amount of two hundred and fifty pounds.