So I can imagine that this was originally a bug. Probably at some point they reached the capacity of things they could physically get out of their warehouse and onto a shipping truck per day, regardless of whether that truck is heading to other trucks or to a plane. So you need to prioritize, every day, the things that
must go out overnight to meet the deadline. If you're full and you have things that could be shipped tomorrow, they wait tomorrow. Then tomorrow, or a week later, or whatever, those things are now on the list of stuff that must go out overnight.
Ideally your capacity isn't filled up by the must-be-overnight stuff. But if you're busy for a few days, suddenly a bunch of regular stuff turns into must-be-overnight stuff. And that delays next week's regular stuff, which then turns into must-be-overnight stuff next week. and there's no way to recover from this automatically. You could recover if you manually added more shipping capacity, or if you told customers that ground shipping would take even longer (so you can spend a few days clearing out backlog), or something.
That said given that the current state incentivizes customers to do exactly what you did, I wouldn't want to be the manager who prioritized fixing the bug....