That's not good. The flexplate is not 'zero' balanced... it has a huge cutout which puts it out of balance by about 4-1/2 ounces. Maybe it's possible the shop made up the difference in the crank, but that's a lot of weight. Putting your original flexplate on is going to screw up their balance job. There are aftermarket flexplates on ebay that 'look' zero balanced, so that might be an option. Best to talk to the shop and ask what the hell they did. Stock flexplates sometimes have clips put on them to fix a minor vibration. That procedure is in the shop manual. Might have been done by a dealer. But Buick originally drilled holes in the flexplate to fine tune the balance.