You should make the bike shop swap it out for the one that was on there. Next time, bring it in yourself, and TELL them EXACTLY what casette you want. FYI, on my racing bike I have 50/34 compact double in front with a 12-27 rear, and on my touring bike I have a 50/34 compact double in front with an 11-34 rear (all shimano casettes, all with 700c rear tires, the tire size also effects your gear ratios). I live in the texas hill country, and tour in the mountains, so I am geared for a lot of climbing. You can run your numbers through the calculator at sheldon browns site to choose your gearing. Pay attention when you ride to what gears you use, what gears you never use, and what gears you WISH you had, and plan accordingly. If you are careful to avoid cross-training, your chain and gears last longer, so the rate at whichh they wear is not just a mileage thing. You should learn to adjust your front and rear derailleurs. Sometimes they just need tuning. A bike is like a stringed instrument. You don't bring it to the music store everytime it goes out of tune, but yes, sometimes parts do wear out.