The customs pre-clearance process is not part of the postal process, so the tendered date would at any rate be Jan 7th (so it's near as nothing as makes no odds). The other aspect, of course, is that our marine/cargo insurers are the ones that dictate policy in this regard, not the post office. If we used the post office's insurance it would be significantly more expensive and they don't cover damage, only loss, and only months after submitting the relevant paperwork to them, which would just add insult to injury, so to speak;)