[canucker2016]

Plus there are some customers who misscan stuff to pay less (they buy a lot of "bananas").

also the discretionary purchases that occurred while waiting in line disappear - no gum/chocolate bar or celebrity/gossip magazine sales

[raverbashing]

The discretionary purchases still exist as the line to the self-checkouts go thought shelves with check-out merchandise

[bombcar]

You used to be standing around bored watching the cashier swipe items, now you're busy swiping items yourself; I suspect the number of gum purchases drop on the self-checkout line (but the longer winding path may grab a few more drinks, but only if it's backed up).