Depending on how far over your bag was, you might send in a note to UA asking for some consideration. Don't expect any and don't criticize the fees as "highway robbery." They are not. You won't find them significantly different on other carriers as carriers charge approximately the same for heavy bags to discourage passengers from using them.

Pity here is that if you knew that your bag was overweight (or likely to be overweight), a second bag would only have cost $35.

As others note, if you were only a few pounds over, it pays to either pull out a heavy coat or boots or to swap some heavy stuff from your checked bag to your carry on and the light stuff from your carry on to your checked.