Suppose the local grocery store offers this option.
Now, whenever* you don't choose the toll lane, you'll be struck by how long the non-toll lines are. You may even wonder if it's a plot to make you pay 5% extra at the register and thus display lower prices on the shelves, or maybe to just waste your time if you don't want to pay the extra 5%.** You don't recall waits at the other grocery store being this torturous, and so you start going there instead.***
*Offer subject to availability and confirmation bias.
**Offer subject to attribution bias.
***Indeed, you're giving other grocery stores a great advertising hook to springboard off of: "our cashiers are fast, friendly, and free!"
The grocery store isn't offering a new, supplemental service; they're charging you for quality service rather than mediocre service. (A more loaded way to put it is that they're ransoming your time back to you.) I suspect customers would resent that quite a bit.
The comment about discounts was because 5% is a lot when it comes to groceries (typical profit margins are around 1%), and charging people just 5 cents per plastic bag causes plastic bag usage rates to drop significantly. It seems like people would go to a different grocery store to save the 5%, and so prices would have to be lowered to compensate- which means that the grocery store is now paying you to wait, which is further from the Pareto optimum.
The new, supplemental service that targets time-starved customers is grocery delivery, which had a number of high-profile failures. The survivors are growing, but it seems likely that perhaps 5-10% of customers are time-starved enough to pay extra. If you only have ~5 cashiers working at a particular time, are you going to put one of them on the toll lane, significantly decreasing throughput?
There's also the question of the time calculation from the customer's point of view. Suppose they save 5 minutes; at $8 an hour that comes out to 67 cents. If the order costs more than $30, that's not worthwhile. (I can't remember the last time I had to wait more than 2 minutes to check out.)
The comment about discounts was because 5% is a lot when it comes to groceries (typical profit margins are around 1%)
I can think of several reasons why typical markup rates would be relevant, but not for why typical profit margins would be. I suspect you looked up "profit" when what was doing the work in your implicit arguments was "markup". 5% then ceases to be thought of as 500% of 1% and becomes thought of as a more reasonable 33% of ~15% or so.
...charging people just 5 cents per plastic bag causes plastic bag usage rates to drop s
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