Based on the scenario you outlined, I would guess that your 4th package was not scanned. For whatever reason, they scanned 3 and decided that was enough.

Ordinarily I would consider that all were scanned and it was taking a day for the scan results to get posted to the website, but the 3 scan results tell me that's not likely in your case.

I make it my practice to get a scan and receipt if the value is high (for me, over $100), but when I see a line of customers I am equally guilty of just placing my parcels on the designated spot on the counter or in the bin. I typically mail 2 or 3 packages a day and don't have time to wait every . . . single . . . day.