In the US, you can insure items for up to $50k when using registered mail. The cost is pretty low, but for a five-figure watch, round trip postage will certainly add at least $100 to the cost of the service. Moreover, just because you can insure something doesn't mean that shipping is something to take lightly, IMO, especially for vintage watches that are not so easy to replace. If an item is damaged or lost, you can make a claim, but there is no guarantee that it will be successful or how much you will receive.
It's easy to be glib when you haven't shipped a valuable watch for service, if the watch is less than $1k, then it's not something to get worked up about obviously. But next time you ship a $15,000 watch (preferably one that you spent years hunting for) for a service, start a thread and let us know how it feels as you are waiting for it to arrive. The mail service is pretty reliable, but if you ship enough items, eventually you will have some major delays, and perhaps some damage or loss.
This is not to say that I don't ship watches for repairs/restoration, or when I sell them. Of course I do. But I don't prefer it.