I'd be interested in knowing how the shop determined that there is only 20% left on the clutch. Unless they dropped the tranny to visually inspect the clutch, at which point they may as well threw a new clutch in there for practically no more added cost, besides the cost of the clutch. If it were me, I'd run it until you know it needs to be replaced. There could be quite a few more miles on it yet.
On the other hand, when I replaced my clutch last year, it was my first time. I was surprised at how easy it was. A lot of labor, but very easy to do. Like Jerry said, one day, two at the most. And I agree...it definitely helps to have an extra pair of hands