If the TOC is responsible for lost tickets we can kiss free postage goodbye.
I think they legally may well be; a supplier is responsible for goods until they are delivered. The courier is simply their subcontractor. They cannot be the customer's, as if they were the customer would be able to choose them, but the supplier chooses them.
This is why I will not post any eBay item I sell by any non-tracked method of postage, regardless of value.
 The only thing I can think of that would make this not the case is that the company is lending the customer an item of their property (a ticket) which evidences a services contract, but isn't quite goods.