If you need to send money through the post, and you don't have a bank account with a cheque book so you can't send a cheque, and obviously cash is unsafe, you can use a postal order. You buy it at the post office, it looks a bit like a cheque, and you write who it's payable to on it, just like a cheque. Whoever you send it to can pay it into their bank account or cash it at a post office.

I used to read about them all the time in stories of children at boarding school. Parents would send their children money this way. But now more people have bank accounts, they get used less than they used to be, and the Post Office charges a large fee for them. It doesn't even allow you to pay for one in anything other than cash any more. The main use I know for postal orders these days is to send money in to a prison for a prisoner you know to use as spending money. They go through the system and end up on the prisoner's spending account much quicker than a cheque, because a postal order is paid for and won't bounce.

But that's what it is - it's a way of sending money in a letter when you've got no other way of doing it.

I have purchased several recently. The cost is less than a dollar (in the US).