I use them all the time for mailing quilts out and I get them in those boxes. You can go to the Post Office's website and do a calculation on the cost of shipping in different boxes or envelopes. I use my kitchen scales to weight small items. I can get a full size quilt, batting and backing (all quilted) in a medium sized priority box -- for $10.70. :)
The last quilt I sent out was 87x87. I checked the FedEx site, the UPS site and the PO site....the Post Office was the cheapest....even AFTER I added insurance and delivery confirmation.
On edit --- I forgot - FedEx WON'T come by my house (in the country) to pick up...only deliver.