I'm not sure of the window for shipping but I've shipped 6-12 week old plants. I think part of the success of shipping is to NOT ship in the heat of summer and try to ship on a Monday so they don't set over the weekend in a hot post office . I also punch a few holes in the box, but I've received boxes without holes in them and the plants were fine.
I ship float plants by USPS in the flate rate boxes . I put each variety roots in a plastic bag and leave the leafy growth out in paper towels .
Now if you try and ship pot size plants it may be hard to do without the risk of the dirt ball breaking plants when the postal workers toss the packages .
I have good luck because the root ball is light and small .