There is a bit of misinformation in this thread...
You are essentially correct, but the rules will vary based on the 2 countries involved. So what I'll outline below is speaking very generally and from my own experiences, since I service watches from all over the world on a regular basis. Note that all this will sound much more complicated than it really is, and provided that the person repairing your watch has experience in these matters, it's really not a big deal. Most of the work involved is on their end, not the watch owner.
When temporarily importing something for repair, there are several things to look at in terms of the possible costs, aside from the actual shipping or insurance:
1 - Duties
2 - Taxes
3 - Other fees
Depending on the countries involved, trade agreements, etc., many items can be imported duty and tax free if they are being repaired and sent back - most countries allow for this. In this case the "value for customs purposes" would be zero on the commercial invoice when the item is being sent to the repairer, which is perfectly legitimate and not an effort to circumvent laws or fees as some are implying. The value for customs purposes is not always the same as the value of the item, and if you use ParcelPro, of course the insured value is never shared with the carrier or any government. This is a requirement with ParcelPro since they want the items to be as low key as possible to avoid theft, so all their packages will have a value of zero on the label, regardless of the purpose of the shipment. Values, if any are stated, will be on the commercial invoices.
In some places the governments will charge the person receiving the goods for repair the duty and taxes up front on the full value of the item anyway, and hold that until proof is provided that the item has been sent back. In this case when the person doing the repairs has the proof that the item has been sent back, they apply to have the monies refunded (the watch owner is not involved in this at all - the proof required is all in the tracking information showing that the item has been returned).
The fees associated with clearing customs (sometimes done by the carrier, sometimes the local government, or by a third party customs broker) are not refundable. Depending on who does the customs clearance, these fees can be significant.
So what about when the repair is done?
When the item is sent back to the owner, what gets charged again depends on the countries involved and their agreements, but generally speaking if it is a warranty repair correcting a fault, there will be no duty or taxes. If it is a paid repair not covered under any warranty, then you may (depending on what exemptions are in place by your government) be required to pay taxes on the value added, which in this case is the value of the repairs. Some countries have quite generous personal import exemptions, and some do not, so if you have to pay will in part depend on the value of the work done in relation to the exemptions allowed.
Because it's been in the news here lately and will I suspect be part of the NAFTA renegotiations that the US has initiated, I do know that the US exemption is $800 (ours here in Canada is $20...and there will be pressure from the US to raise it).
I hope this helps clear up a few things.
Cheers, Al