Originally Posted by FBinNY

yes and no.

Changing a chain is simple enough and often all that's called for. But the shop was simply giving you fair warning of the possibility that the new chain might skip on a worn cassette. This is a common problem, and discussed at length here on multiple threads.

The shop rightfully felt obligated to warn you to the issue because IF they installed a chain and IF it skipped, and then they told you you'd need to shell out another $70, nobody would be happy.

FWIW - the potential skipping issue can be predicted, with some cases near certainty that it will skip, or that it won't. But there's a wide band of gray where it MIGHT skip, and the only way to know is to fit the chain and ride.