Any simple checksum methods for CC receipts with tip?

Factual Questions
1

I’m always a little worried when using a credit card at a place where there is a tip line and the server has to manually enter the total amount into the computer. It is easy for the value to be enterred incorrectly either by accident or on purpose. I’m wondering if there’s an easy checksum method I can use to catch these errors when I view the entry on my CC bill.

For example, say the food+tip is 27.53. An easy checksum is to add up the digits except for the last one (27.53 = 2 + 7 + 5 = 14) and then replace the last digit of the total with the last digit of the checksum (4 in this case). So I would change the total to be 27.54. Now when my CC statement comes I can easily validate that the amount wasn’t changed by redoing the checksum calculation and verifying the last digit matches.

However, the tough thing with this scenario is that I can’t just change the total line. I have to change the tip line so that the total comes out correct. If the original scenario is this:

Food: $22.94
Tip: $4.59 (20%)
Total: $27.53

I can’t just change the total from 27.53 to 27.54. I have to actually change the tip entry. But if I change the tip entry, the total changes and I have to come up with a new checksum digit.

One initial way I thought would work is to leave the last digit of the tip blank and fill it in after computing the checksum. For example:

Food: 32.01
Tip: 6.4_
Total: 38.4_ 3 + 8 + 4 = 15 —>> checksum = 5

So with a checksum = 5, I replace the blank with the number so it adds up to 5:

Food: 32.01
Tip: 6.44
Total: 38.45

But, this only works if the ones digit increases. If it needs to decrease, I can’t use this method.

Food: 32.07
Tip: 6.4_
Total: 38.4 (checksum is still 5)

I can’t replace the _ in the tip with a number so that the total ends up with a 5. I need to increase the tip by .08 which changes the tens digits which means I’d have a new checksum.

So, is there a simple checksum method I could use? It needs to be a simple method I can do in my head without having to correct any amounts I may have already written down.

2

I don’t do this myself but someone told me they round the total to the nearest dollar and put a line through the tip. So in your first example, the total would be $28.00 and it is up to the restaurant to calculate the tip.

Your “checksum” would be all totals should be integers. However, this can be circumvented if the restaurant adds an extra dollar.

3

How about looking at the receipt when they bring it for your signature? And/or saving the receipt to compare to your statement?

I’m not sure why all of these mathematical gyrations are needed.

4

Similar to AnalogSignal, just do the short math to work out how to get to a round number, like to the dollar of half dollar.

5

Normally at restaurants where you are served at your table, they take your credit card and bring back the bill. You then add a tip to the bill, compute the total, and sign the receipt. After that you walk away. The amount doesn’t get entered into the computer until after you’ve left. Maybe things are different in Wisconsin?

Anyway, to the OP, I’m pretty sure it doesn’t matter what you write in the tip line, they have to use the total. So just apply your checksum to the total (maybe scratch out the tip line to be sure).

And I agree that your checksum method is better than using a round number, since if the server changes the number (deliberately or not) it will often still be a round number. Whereas with your method it is very unlikely that the checksum will still be correct after a change is made (impossible if only one digit is changed).

It’s a good idea but I don’t think I’ll bother with it. Imagine trying to explain your checksum system to the CC rep when you call to dispute the charge :rolleyes:

6

Out of curiosity, have you ever found an actual error in an actual statement?

7

nvm…

8

Have you ever notice that they always bring two receipts from the credit card doohickey? One of those is for you. Granted, you’d have to go through the extra effort of writing your tip twice so you can compare your copy to your statement later, but if you’re going to do that anyway, it’s certainly easier than computing a checksum.

9

What I was trying to say…

You’re ignoring subtraction. Whatever your target digit is, you can add or subtract some number of cents from your tip to get to it.

So, for 12.43+2.46=14.89, don’t add 0.04; subtract 0.06.

10

I disagree that writing out everything twice and then keeping track of all your receipts until you get your CC statement is easier than adding a few single digit numbers together. The checksum system is much easier. All hail the checksum system.

11

Just checksum the dollar portion. This is the important part anyway. I am too tired to figure it out, but I think this reduces your issue.

I did a search and found this article suggesting the same.

12

And yet, in the article linked by CaveMike, the first step that they recommend if you notice a checksum problem is to:

Seems easier to just keep the receipts and check them, without worrying about working out a checksum.

And if you don’t keep your copy of the receipt, what evidence do you have that anything went wrong in the first place. Do you think the restaurant or your credit card company are going to find your laborious explanation of your checksum as convincing as a copy of the original receipt?

Maybe i’ve just been lucky, but i check most of my credit card receipts pretty closely against my statements, and i don’t recall ever being fucked over on a tip by a restaurant server. I’d ask the OP the same question as Exapno Mapcase: has this actually happened to you? Or are you looking for a solution to a nonexistent problem?

13

Plus even if you have your copy of the receipt, the tip portion would just be written in pen, by you. How could you prove that you didn’t just write a different tip on that receipt?

14

Is this really worth your time? I’ve worked in restaurants, and I’ve never heard of fudging tips- low tippers are a daily thing and you just figure it will even out in the end.

Even if one in a hundred times you catch someone cheating you out of two bucks, your return on that per hour is going to be astonishingly low. Certainly if you follow it up, the phone calls to the (likely unamused) police and such are going to push the actual amount of anything you end up recuperating even lower. It seems silly to get all vigilante over something you would have never even noticed, when you probably get willingly ripped off all the time (by movie theater popcorn, or whatever.) Sure, it’s the principle of things, but of all the baddies in this world hunting down waitstaff that fudge tips doesn’t seem like a top priority.

15

It seems to me that, if you are the type that needs to know that every cent is properly spent, then you should be keeping a checkbook. Write down how much you spent in there, and if it’s wrong later, dispute the charges. It’s a lot easier with credit cards than checks, as the process is speedy and the card owner is usually assumed to be right (as I learned on this very board).

16

Pay the bill by credit card and leave the tip as cash.

I was a hotel auditor for years and we always double checked the math on resturant charges for tips. If the server was going to pull anything, they wouldn’t get away with it more than once or twice, unless you had a auditor and accounting department in on it as well.

17

And many restaurants bring a terminal to the table where the server has entered the amount and you can enter the tip as an amount or as a percentage before approving the transaction. Much better method IMHO and reduces the need for impromptu calculations.

18

That’s how I’ve always done it.

19

I do keep all the paper receipts. The checksum would mean I wouldn’t have to necessarily verify each paper receipt. I could look at the statement and find the modified entries and then dig through the saved receipts to verify.

I have found mistakes in the past by verifying paper receipts. The CC company can pull the original paper receipt if there is a question about it.
A second part of my question is how to come up with a good checksum. The mistakes I’ve found in the past have been where two numbers are switched. So 23.15 gets entered as 32.15. My simple x+y+z checksum isn’t so good for that type of error since the checksum is the same regardless of the order of the numbers. My checksum for 123 is the same as 321. Is there a simple checksum I can use that will be different if numbers get transposed? By simple I mean an easy math transformation I can do in my head.

20

Well I guess you could swipe out the tens digit when doing your calculations so it’s 45.61 because 6+5=1 and if the switched it to 54.61 you’d know instantly.
But that wouldn’t solve problems like 34.37 versus 43.47 or 53.58 versus 55.38 or any number of other scenarios where, again, the best recourse if you’re truly worked up over the situation, is to just save your receipt compare the numbers on your statement to the numbers on your bill. I’m not sure why we need an elaborate system to prove whether something is happening when there’s a simple solution already built in to accomplish what you wish to do.