A notice would have been left if the customer didn't accept the parcel - perhaps it was too big to fit in the letterbox and had to be accepted by hand, or required a signature and no one was home. A notice is usually left asking them to pick it up, but occasionally the notice is forgotten, or lost. I purchased some marshmallows off of etsy at one point and they never arrived. I told the seller and followed their protocol (wait x number of days then they file a claim if they can't track it), and they told me it had been returned to them because I never accepted it. I never got a notice from my postman like I usually do - if I had, I'd have scheduled a redelivery for the soonest possible date, but in this case the postman made a mistake.
If your customer was nice about it, she might be willing to wait for them to be reshipped once you received them, but at the same time, you have to ask yourself if you'd be able to sell these slippers to someone else. If they're all custom made, would you be able to make a reduced price listing for this pre-made pair? If so, I would suggest remaking them right away. This original pair may end up getting lost, and your customer will be waiting for quite some time if you wait to receive them.
