Do I know anybody that has shipped their own parts to JLCPCB for use in PCBA? (https://jlcpcb.com/help/article/how-to-consign-parts-to-jlcpcb) In particular getting the paperwork right. It needs an invoice for the parts but I'm not actually selling them to JLC, they are my parts which they will be sending back when they have soldered them. It's all a bit confusing and getting parts *in* to China isn't something I've done before.
How to consign parts to JLCPCB

Learn how to consign your electronic components to JLCPCB for use in your PCB assembly orders, as a part of their Parts Inventory Management system.

@rc2014 it works like any other regular import, you need to declare goods at their commercial value, what that value is is up to you but should reflect a reality if you don't want customs to dispute your declaration.
You want to use a logistics company that does import brokerage. E.g. DHL, FedEx, UPS. Otherwise you need another company in China to handle import for you.
@timonsku Thanks. Do you know if that value is what JLCPCB put on the form when the parts are shipped back (and attached to a PCB). I assume I will be paying import duty as they go in to China, and once again when they come back out.

@rc2014 it should be declared as 0 for your order but I never checked that in detail, could very well be that they declare your imported value again. Doing customs for transit parts (e.g. meant for not staying in the country for general use), is complex and not worth it for small qty.

Generally I would avoid shipping parts manually like that unless you have a giant reel of very high value parts.