Marking an item as "Posted" is just an admin thing and will not be taken into account in the event of an INR claim.

What they are looking for is proof of lodgement showing the tracking number and it being date/time stamped as lodged at a PO.

In the case where a tracked item is lost (never delivered) then this lodgement scan will be the only scan event to show on the tracking status. If you do not have that you will lose the INR case. This is exactly why you should not drop tracked items into red street boxes (or yellow if express), as you don't have this lodgement scan event.

So if your local PO is open on a Saturday morning (and some are) you could take your items in and have them scanned in as lodged on the Saturday. In reality tho it will sit in their back room until Monday when it will go to the nearest distribution centre. But you would be covered as it was clearly lodged within 24-hours if they ordered on a Friday night.