Hi everyone! I was wondering how you guys go about shipping large flattened lego boxes. I just got a bunch of modular ones and others and I'm just wondering how to ship one or so just in-case I ever need to sell one on bricklink or ship one to a friend. Hope I posted this to the right forum. 

You basically need to mail the box inside another box to avoid crushing or excessive bending. You can either unflatten the Lego box and fill it with reasonably dense packing material and then put it snugly inside another box just a bit larger, or leave it flat and put a few layers of cardboard sheets on either side of it to help keep rigid, then do something like packing peanuts or a lot of scrunched paper to fill out the shipping box.  Best way to go is to just get a proper flat mailer like these:

https://www.uline.com/BL_1303/Jumbo-White-Fold-Over-Mailers

Either way, I would suggest bagging the Lego box as well even just a kitchen garbage bag wrapped around it to protect from any incidental moisture or dust & bits from the packing material.

Keep in mind that sometimes mailing it in a larger standard cardboard box like the way Lego ships the modulars if you order one is actually cheaper than a flat mailer because the box isn't an odd size, but that's easy enough to check by putting in the package dimensions and weight on any of the shipping service websites.

I wouldn't mail one flat. If the outer box is flat, there is a good chance it will get folded or creased. As above, use a regular LEGO shipping box for the outer protection.

Thank you very much , @MAB and @deraven for your wonderful advice! I'll see if I can find some mass quantity of boxes for hopefully a semi-cheap price at my local supply store. Last question I have is what the cheapest option for shipping one would be for someone in USA. I don't think I can ship this as media, and I'm hoping to pay less than $15 if I do plan to ever ship one out. Thanks again you two, I appreciate it. If either of you knows the best carrier to ship one is, I'd be more than happy to hear what you have to say. 

USPS, insured & tracked is probably the least expensive in most cases.  I'll come down to the size of the box more than anything since it's not going to be heavy.  You can go to USPS, FexEx, or UPS websites and plug in the box dimensions, weight, and to/from Zip Codes and it'll tell you the price so you can compare.  

All right, thanks @deraven! I thought as much,  but I wanted to make sure there wasnt some option I was missing just incase. Thanks again. 

When I was Moving from Boston to Alabama, I hired a small truck and one man to help me transfer my large boxes of lego along with the other stuff I had. I could, for sure, drive on my own if only I had a car. But a couple of years ago I divorced my ex-wife and lost the opportunity of driving my auto. So searching through google and by finding a particular https://starkmovers.com/moving-from-boston-to-alabama/ link helped me a lot. First of all, my time and nervous system were saved. 

Thus, I successfully transferred my Death Star and two Star Destroyers of Lego without any problem. They were in carton boxes. 

But as deraven mentioned above, you may try to ask ups for help as well. Or FedEx. 

Should you decide to send anything flat, sandwich it between two sheets of 10mm thick foam board before putting it in an outer cardboard box.

Foam board is light and difficult to bend, but easy to cut to size. It’s expensive if you buy it from art supply stores as single sheets, but can be bought cheaply online in multipacks.

When I moved, I had to order a large truck and two movers in order to successfully move. They packed all my Lego sets in special boxes, thanks to which they successfully moved to a new home :)
I can recommend this company – I worked with them twice and both times there were no problems.