DHL doesn't do domestic (within the US) shipping anymore. That's what they say anyway...the truth is that they do, but it's very limited so for all intents and purposes they don't ship domestically. They have a huge presence overseas, and may be competitively priced.

And it's mostly the box size, as others have stated. I worked for DHL designing and supporting their sort centers, and the extra charges for the oversized items are because they're too big for the the automated handling at the sort centers and thus have to be sorted/handled by hand.

And from my experience, I can suggest the following: packing tape is cheap...don't scrimp when taping your box. Also, assume that something will hit your box from all sides and pack accordingly. The package following yours on the conveyor system could be a 75-pound box of bolts that is the size of couple of tissue boxes. Your large box with non-reinforced sides won't stand a chance against it...

Charles