I really would let it go. I know you're not annoyed at your friend, or wanting your money back or anything like that (well, I hope not!) bu I just don't think there's anyway to raise this, however nicely, without making your friend feel bad and possibly feel she had to offer you £1.09.
I know your motivation is just to ensure it doesn't happen again but really, how likely is that?
NB if it was a company did this, or a school like a PP mentioned, I'd be down on them like a ton of bricks. But a friend who'd done the lovely thing of sending me a birthday card? No way.
PS exactly the same thing happened to my MIL on Mothers Day when she didn't get a card from her daughter. A LOT more than £1.09 was spent on sobbing hysterical phone calls to DH, and then from him back and forth to SIL, before the matter was cleared up. We felt a bit grumpy at the Post Office on that occasion