I just went to US Post Office expecting to pay around 16 dollars to mail a plant Priority Mail and dropped my jaw when the man said 53 dollars!

Dimensions of box are 9x9x36 less than 10 pounds.  

What is the best way to mail plants over 28 inches tall?

I did receive a fig tree several years ago that was shipped standard, about 8-10 days and it was fine, what do you think about standard shipping for a plant?

Thanks!

I own a small nursery
I ship bare root and ship Priority Mail
anything over 25 inch's and under 37 inch's are shipped in the following box you can get them free shipped to your house or pick up a few at podt office.

https://store.usps.com/store/browse/uspsProductDetailMultiSkuDropDown.jsp?categoryNavIds=shipping-supplies%3afree-shipping-supplies&categoryNav=false&navAction=push&navCount=0&productId=P_O_1098M&categoryId=free-shipping-supplies

I ship in the spring and fall
I take the tree out of pot  and shake off all the dirt then I dip the roots in a bucket of water and shake to get as much of the dirt off.
then I wrap the roots in paper towels dip in water then squeeze  it so it is not dripping them put the roots in a plastic bag and tape the top around tight so no water can leak out, then I wrap the top in newspaper put it in box then put newspaper all around it so it does not move in box .  I ship on Mondays or Tuesdays so it gets to the buyer before Sunday and does not set in a hot truck over the weekend. I have a scale and weigh then at the nursery and print shipping labels thru post office when doing it at home you save on postage.  

Don once you get over 28" the price goes way up Try fedEx they are usually half the price 

using the priority mail 2 - 3 day box I linked to at 10 pounds shipped from say me to Dallas zip code would be $14.35 if you took it to post office and they weighed it and you paid then. now if I weigh it at home and pay for postage online and print it,  it would cost $9.56...

I think the PO is using the pricing by zones. Within a zone, it is the cheapest. The most expensive is to ship from coast to coast.
The price can be checked online and, as mentioned above, it is cheaper to pay and print at home.