Is there some kind of an initial "scan" upon drop-in to the box and then a second scan when they pick it up?
If not, then I imagine the metrics would only be discoverable from USPS themselves, which of course, they are going to muzzle.
You would need to search the tracking number and see when the first scan occurs. Shippo is only looking at shipping label creation date which could be offset quite some time from when the package gets scanned. Large businesses might have enough employees to where they can pack, label, and ship out an order that day but small businesses might have someone hand delivering packages to the post office every other day.
Example one of the ones I got dinged on:
Jun 30, 2020 7:29pm Tracking number provided
Jul 3, 2020 6:45am Accepted at USPS Origin Facility BEAVERTON, OR 97078
It was dropped off Jul 1 in the early morning. Ended up sitting there for two days before they got around to scanning. Admittedly it has gotten better the last couple weeks but it can still take them a day.
The easiest way around this though is using a scans form which is just a printout where they can scan once and it marks all of the packages as accepted at once. It still takes time to prepare the form, print and find someone at the dock or drop window to scan it - they don't seem to particularly like doing it.