Check the USPS website for free priority parcel boxes.
There is an assortment available which they will deliver to your doorstep.
I ordered the two different sizes in triangular boxes which I have found extremely handy for posting small trees etc - don't have to trim roots or tops.
What you don't need, your local PO will most likely take them off your hands.

I think this may be a new thing since the postal increase this year. When I took a large Priority Mail "tube" (triangle box) to the post office yesterday the worker measured it despite it being one of their own boxes!

I've found out a little more about the extra charge which is called a "Balloon Rate". The 28x28 inch limit that I mentioned is wrong. The box is measured by adding the circumference plus the height. A square box that is only 18x18x18 would still be charged an extra fee because the total inches would be 4 times 18 which equals 72 inches (the circumference or girth) plus the height (18 again) which would be a total of 90 inches. If the total is over 84 inches they add on the extra balloon rate.
The clerk I talked to said they measure the largest part of the box to find the circumference, then add the height. I can't find this information anywhere on the USPS website. Only that the total can't be more than 84".
On a rectangular box, it could make a big difference which part is measured to find circumference.
I've used those triangular boxes in the past also but it wouldn't work this time because the roots were sort of horizontal and they wouldn't have fit in there. I do think I'll stick to using priority mail boxes from now on. I've also been checking out those flat rate boxes that someone else mentioned.
By the way - The box I mailed is still somewhere between MO and NJ, mailed Monday, March 27th. I've been checking delivery confirmation daily. I think it's a lost cause now.

Windchime - I haven't heard from you. Hope everything is ok. Have you tried to email me?

I've been able to fit a lot of plants in smaller boxes, this time of the year when they're or should be growing so fast they can use some trimming off the tops.

good luck,
GB

Christie, they DO charge an "oversize" fee, just like FedX and UPS. But I've found it much cheaper to send big boxes by FedX Ground.

A box that would cost me about $30 by Priority would only run about $11 or so with FedX Ground. AND in most cases, it gets there in 4 business days and it's insured for up to $99 for free.

Also the tracking is free. They also have a better tracking system. You can watch the package every increment of its trip and see where it checks in along the route. You can't do that with the USPS delivery confirmation tracking.

I use the post office only for smaller packages. FedX and UPS are geared more towards larger, heavier packages. And their rates are much better, with better service.

Flat rate boxes are rather small. You really can't get a lot in them. There are only two sizes for flat rate, and in most cases, the stuff I mail won't fit in them. Check the dimensions at the USPS site. I was rather disappointed. I often need to send stuff in a #7 box, but that's not a flat rate box. One of their flat rate boxes is about half that size. The other is a little bigger than a shirt box.

NancyAnn - Thank you! Great information! Next time I will know to go to FedX if I have a large box. I do wish USPS had better tracking. I'd like to know where my box has been traveling the last two weeks. lol
And guess what! It finally made it to NJ. Yippeee! Two weeks it took them! I got a email from my trader today and they are actually showing signs of life so maybe they'll make it after all. I think most plants wouldn't survive such a rugged trip but dormant shrubs like to be dark and cool. The perennials that were in there aren't quite as happy about the wait. : (