Bizarrely you can't track something on Royal Mail (presumable signed for…) until it's being delivered.
There's no guarantee that even first class will take a couple of days to arrive.
Alas, you'll have to find some shoes locally. And of course the ordered ones will turn up on Monday. Then you'll have to stump up to send 'em back.
Think jam_bo is right, it's something like 14 days before it's lost – and the seller has to initiate the claim. They may refund you straight away, or may wait until they get their money back (in case you're pulling a fast one).
Worth a trip to the sorting office? Maybe they've tried to deliver but postie forgot to tell you?