Without full information I don't think anyone can make a full and proper judgement. I wild guess perhaps.
However one wild guess would be if it was dispatched by the repairer to you and the post Office have lost it, then it is between the Repairer and the Post office. I would have thought that the repairer would have used a special delivery parcel where it had to be signed for on delivery and it can be tracked where it went to from the time it was left with the PO to the time when it should have arrived on your doorstep. If the PO admit the lost it then they owe compensation to the repairer who in turn should pay you.
Incidentally who is the repair firm?