100 miles isn't much! Worth a try to get the right size. Tell them they sold you the wrong size. Tell them you looked up on a fit calculator online. They sold you the wrong size which is not good coming from a shop. Tell them you learned that they had sold you the wrong size through bikeforums online. IF they don't or get pissy, the whole world will know about it! Negative reviews online!

Some shops have a 30 no Q asked return/exchange poilcy for that reason. Who knows!