What about a $100+ grocery transaction? Are people willing to wait around 10 minutes in front of the cash register to wait for a single blockchain confirmation?

There isn't any real risk of double spend attacks. The effort it takes to make a double spend happen requires far more money than that (and no, I'm not taking about a 51% attack), and is not reliable in any way.

A transaction is broadcasted to most of the network, and certainly to the main pools within seconds. So once you see a transaction in the pool of trusted miners, you can assume it's okay. The only "real" possibility of it being a double spend is if they appear on the network at the same time, but is visible when the second transaction uses money that the previous transaction have spend. At that point it's equivalent to the credit card failing from an interruption in the connection - you just try again (the oldest win).