Yeah, that’s what I thought this thread was going to be about, too. I was relieved to discover that the OP isn’t an idiot.
Thanks!
My exact thoughts as I was about to mouseover the thread link were something along the lines of, “What kind of moron doesn’t understand why coins are more robust than bills?!” 
(I’m guessing it may be related to the kind of moron whose brain likes to insert words into sentences to give them new meanings.)
I’m glad I wasn’t the only one!
My first job was at a super market… this was probably in1994 or so… and they gave us explicit instruction to always place coins before bills, because it was eaiser for the customer to handle the money this way.
Now I’m back in retail, and I am still in this habit. I give them their change, which they usually put right into their pocket/purse, and then I hand them their bills.
It amazes me that a LOT of customers will do one of the following when paying:
Hand me crumpled up bills that are in no particular order.
or
Hand me WAY too much money. At LEAST once a week I end up handing a customer their extra $20 back.
I do just the opposite: two pennies, a nickel, two dimes, two singles, a five, and a ten. The coin part of my wallet is small and skinny, so I like to use my change first. So if I pay with the bills first, I’m probably gonna get a lot more change that I don’t want or need. I’m usually able to pay the exact coin amount.
The Squidette and I have discussed the phenomenon of coins on top of bills a few times, and decided that it’s a method that’s most conducive to women getting their change back. The fact that the overwhelming number of cashiers are women leads to that method.
It’s all got to do with the formation of mens versus women’s wallets, see?
Women’s wallets are designed to flop about in purses, so they’re big enough to handle change. There’s that little snap opening thing-a-ma-bob that splays wide to accept coins, and a separate flap for bills. When you get the coins on top of the bills, you can use the bills as a trough to slide the coins down into the open coin purse, and then can place the receipt and bills into their necessary sections in the foldy part of the wallet.
Men’s wallets, on the other hand, have to fit in a pocket, so the coins must go loosely into a pocket. When guys pay, they whip their wallet out, and stand around with it open in order to put the bills back in when they’re received. If the coins were delivered first, you could cup them in your hand while you put the bills into the wallet, and the wallet back in your pocket. At that point, you’ve got a hand free to drop the coins into your pocket, where they can stay safely nestled until you drop your trousers into the laundry hamper, at which point they become the property of the Squidette. When the coins are on top of the bills, you’ve got to monkey around by trying to transfer the bills in to the hand that’s holding the wallet before dropping the coins into your pocket. This usually winds up with half of the coins spilling on the ground, and the man looking like an ass while trying to recover his lost 23 cents.
It’s all a gender conspiracy, see?
Somebody already brought up this “theory.” I already pointed out that it doesn’t apply to me or any other women I know. I don’t want my change on top of the bills where it’s “easy” to slide it into the change section of my wallet–it’s just as “easy” for it to slide all over the floor. I want the coins in my hand, where I can then easily dump them into the change section of my wallet *without *the risk of them ending up anywhere else.
The explanation CookingWithGas offers looks plausible to me because that’s what I also observe:
When people give me change without having calculated it on a cash register they usually count up - e.g. when a taxi driver gives me change for an 12 € fare on a € 50 bill, he will give me €3 in coins - making 15 € , then a € 5 bill - making 20 €, then a € 10 bill - making 30 € - then a € 20 bill - making 50 €
On the other hand when the change is displayed explicitly on the register they will usually go from large to small.
Cashiers do seem to be reasonable numerate too even nowadays - when I have to pay for a 4.82 € item and hand over 5.32 € in most cases the cashier immediately understands what I am doing that for. Only very rarely do I get a blank look.
Sorry, I didn’t see it. Well, it works for at least one woman, I suppose. The only working theory, I guess, is the aforementioned suggestion that change is decided automagically by the cash register instead of by counting up, and… it’s easier to take the bills out before the change?
We read numbers left to right. When you see “$17.37” as the change to give, it makes sense to take out the $17 and then the $0.37. That doesn’t mean you have to hand it to the customer in the same order, though.