DEN:
This is straight from the "horse's mouth" as my missus runs our local post office; This is what should have happened:
When buying a postal order, the clerk will ask you if it is payable to anyone in particular. If yes, clerk will type in the person's or firm's name.
If no, it will be left blank with the 3 stars as yours was so that the purchaser or payee can put the relevant name in.
The fact that she chose to send you a crossed postal order, unfortunately meant that you could not therefore cash it at your post office, but meant that it had to be transacted through a bank or building society and paid into your account there.
The staff at the Halifax should have been able to do the transaction but it's probable that their confusion was due to lack of knowledge that they are entitled to cash the postal order for you.
You would still have had to insert your name where the 3 stars are, however, the Halifax staff, had they been clued up, should have realised that and advised you accordingly.
A crossed postal order is a very safe method of receiving cash as it cannot be cancelled or stopped.
Did you insert your name? This is important because if you just left it with them without your name on it, technically anyone could then put their own name on it and send it to their own bank account. It is quite in order for you to either have inserted your name yourself or for the sender to have done it, or for the Halifax staff to do it for you.
Hope this makes sense. Come back if you have any more queries, but it seems as if the trouble could have been averted if either the sender had put your name on it first, or if you'd contacted a post office instead of a bank.
We all live and learn, don't we? Hope this helps.