you should not need shims, if you do then it is not the correct starter.
duralast is a piece of **** brand, but what can cause the starter to fail is inadequate voltage to the solenoid on the starter. The solenoid on the starter draws a decent amount of current, and if the starter relay is faulty then that can cause new starters to fail, what'll happen is the solenoid will burn out and not force and hold the pinion out to engage the flywheel. So try replacing the starter relay in the fuse box under the hood, it will be one of the big square gray relays. And the only thing that really ever fails on the starter is the solenoid which unfortunately is not really replaceable by the consumer... the starter/alternator business wants you to turn the whole thing in and buy a whole new starter for > $100 while they replace a $10 solenoid and then resell it.