> On the other hand, the cost of maintaining extra self-checkout machines costs very little. So in practice, they tend to be available to handle spurts of activity

If only! At the (UK) Tesco metro near my workplace, the staff have this insane habit of keeping half or fewer of the available self-checkout machines in operation. The rest of them have a big symbol like a stop-sign and are unusable.

Only if a big queue builds up will the single attending staff member go round and laboriously make the other machines available, whilst simultaneously having to attend to the frequent weighing issues caused by the machines. By the time they're done with this the lunchtime rush is practically over.

I mentioned this to a work colleague who suggested the reason is that they want to reduce the amount of time counting cash out of the machines at the end of the day, so having fewer machines in operation is preferable.