I travel a lot.
Not all countries have the same standardization and cash controls as the United States or Western Europe. In most of the world, actually, everything is pretty ad hoc.
Part of this means, at least 5-10 times per year I'm having someone hand me the wrong amount of change or otherwise screwing up billing pricing.
Funny enough, no one ever hands me too much change. No one ever accidentally marks a bill as paid that isn't paid.
But the reverse happens. Often.
There's basically no adverse consequences to the person doing this. They ask you for more money or give you too little back, and they just blush sheepishly if you call them out.
What I do is calculate the change I should get before it's handed to me, and then it's easy to check. Also, I'm especially careful when someone hands me a weird mix of bills (if I'm getting back 10's and 5's at all when the change is $81, something might be up).
I actually don't mind the mental overhead of calculating before getting money - being able to do basic arithmetic fast is probably the most useful math skill for everyday life, and it keeps your arithmetic sharp.
Sometimes I wonder if there's a way to implement some kind of deterrent to doing this sort of thing, but I can't think of a way that's not asinine. So I keep doing the math before getting the change, pointing out that three 20's and one 10 doesn't add up to 80, and puzzling afterwards about if there was anything else worth doing there.
Funnily enough it has only happened to me once in my life, and it happened in Australia. The meal was $8, I paid with a $10 note, and received a $20 note in the change. Being the gentleman that I am (some may call this stupid), I gave them the change back. However, it took me about a minute to explain that they gave me TOO much change, they just thought I was complaining that I got underchanged!
I used to work in my family's shop, and at least once or twice I accidentally gave too much money. Luckily, the customer is nice enough to point it out and return the cash.
What's more aggravating is when a customer would lie and say they gave me a larger bill like a $20 when it was really a $10. We had a rule that we always give the change back first before putting the cash into the register because of this. And I also learned to do mental math pretty quickly.
Traveling twice to Romania this year, I noticed there that usually your total is rounded up or down to the closest unit. That way shops and people only handles bills and no coins (the smallest bill (one leu) is about $0.3).
Its interesting you go through all that, I've worked in retail and one of the things we used to do was round down the price so people didn't have to worry about change. If something came to $15.30 or something similar we would call it $15 even and customers would think you did them a favor. In reality 30 cents wasn't usually anything since we probably paid like three or four dollars for the item. People don't want to have to count their change to make sure its right and it would appear foreigners try take advantage of the "lazy or stupid american" far more often then you'd like.
For me after a few years working in a gas station out of high school I learned to count backwards on change so its actually more of a second nature thing to me to know exactly how much I'm getting back before they've opened the till. In your case do like my grandpa would do, carry a change purse and enough ones and fives so that you always had exact change.
Also considering what part of the world your in, I don't see why you couldn't apply my retailer rule the other way around, they ask you for some weird price, round it down reasonably and tell them you'll give them that much. I betcha they'll bite, when possible everyone likes to haggle, see yards for a fine example of this, it gives them the feeling they are in control and a sense of satisfaction when they've made a deal.
Edit: I gave up on financial goals in late 2011 after some huge financial and artistic wins... money shouldn't be taken too seriously. For the record, they were all basically on track, some were being massively exceeded, others were a bit behind schedule, but were all happening.
I set my next 10 years of financial goals on June 28th. That was exactly a month ago.
1 year - Critical Thinking [my first book] out. Blog income trickling. Some info products. Some freelancing. Something else, some X-Factor thing bringing in cash. Net monthly income positive. Health insurance. $50,000 in the bank. Expenses = income per month minimum.
3 years - 3 to 5 books out, many products out, blog income robust, some working on big exciting deals. $10,000 per month total, $5000 passive at least. First property owned. $300,000 in the bank.
5 years - 7-10 books out, many many products out, many passive income internet properties, working on big exciting things, $50,000 per month total, $40,000 passive at least. $1,000,000 in the bank.
I wrote about becoming a pro poker player a couple weeks ago. I was going to write about something else, but two things sap my motivation:
1. I have wicked bad allergies to something, probably the Cedar-Elm, and can hardly focus on anything for more than 15 to 20 minutes before I start rubbing my eyes and sneezing.
2. All I really do now is play poker, so it's on my mind.