Your shop ripped you off unless you badly abused your bike or there was other physical damage. I would NEVER go back there.
A chain and cassette will require replacement periodically and you have two ways to approach it. Either run them until the chain is "stretched" about 1%(it will measure about 12-1/8" over 24 pins while a new chain will measure 12" exactly) and then replace both. Or replace the chain every 1500-2000 miles and get three or more chains per cassette. I do the first and get 6000 to7000 miles on a chain and relatively low cost cassette like a 105 or Veloce. Chainrings last for much longer with over 30,000 miles being common even with my 6000+ mile chain life.