Grocery shopping is regularly over $100. A single network confirmation typically takes 10 minutes, but can take as long as an hour. (Block 152218 took 1 hour 40 minutes). Of course, there are plenty of examples where a blockchain was calculated in seconds... but such is the random nature of waiting for blockchain confirmations.

A quick back-of-the-envelope calculation says that 30% of the time, blockchains will take 20 or more minutes... and 5% of the time, blockchains will take 30 or more minutes. Are your customers willing to wait 30 minutes 5% of the time while doing their groceries?

Hell no. Waiting for even a _single_ blockchain takes too damn long, even if you reserve it for substantial amounts of money like $100+ transactions.