From LFS website:
The payment methods we accept are credit card (VISA, Mastercard and UK debit cards), Paypal, Moneybookers, UK Cheques and Money Orders. More information can be found on the 'Buy a license' pages (make sure you are logged in).
Hmm, it doesn't say Postal Order exactly, but I am pretty sure it is a valid way of paying. I think you will need confirmation from the developers on that point. Maybe it needs ammended to say Postal Orders as well, as it isn't crystal clear if they are the same or not.