I'm flying united tomorrow across country and was wondering about the baggage fee cost?

I just wanted to check if anybody else had any experience with getting charged with the oversized baggage cost. When I was filling out the online form I put I was going to check the board, but when I selected it was a larger item it tacked on. An extra 200.

I figured I would try to just check it in as a regular item but wanted to see if anybody had any first hand experience since the box is not too irregular?