Congrats on your new wheels. If you're in the U.S., the Chevy/GM dealership should not charge you for 1 top tether anchor retrofit based on the info. in the most recent 2015 edition of the LATCH manual. GM dealerships are to provide 1 top tether anchor retrofit free of charge (no charge for the part nor for the installation labor, entirely free of charge to you) in retrofittable 1989-2002 model year GM vehicles in the U.S. per GM Service Bulletin #99-09-40-004a dated April 12, 2005. Additional tether anchors anchors beyond the first free one would be out of pocket, but hopefully not excessively costly (get a price quote in writing in advance to avoid any surprises, though) considering it shouldn't take much longer to install another one or two top tether anchor retrofits beyond the first free one. Dealerships unaware of the GM retrofit program/policy are unfortunately a recurring theme here at times
but you can refer the dealership to that relevant service bulletin number for details to get the dealership up to speed, and enlist the help of GM corporate if need be to nudge the dealership into fulfilling the first free retrofit.