Because the bill included a detailed breakdown of all the fees by type and transaction. Typically our clients were charged a fixed monthly fee, a fixed authorization fee charged every time a payment was attempted (even if it was declined), and a fee applied to successful payments that was a percentage of the payment amount. There were other fees for things like processing chargebacks, but IIRC they were the same for everyone.
> If the month's charges were 406.783228 and I get a bill for 406.79 then that seems perfectly good.
Yeah, we definitely weren't allowed to round up and keep the change. I can't claim to know all the details involved but I suspect that doing so would've at least violated our contract with the payments gateway. Might've actually been illegal.
> If I get a bill that says 406.78, plus a separate 0.01, that's weird.
That's not how it worked. For the sake of simplicity let's assume that your activity is always the same, therefore you have new charges totaling exactly $406.783228 every month.
* Month 1: You owe $406.783228, your bill is $406.78, a balance of $0.003228 rolls over.
* Month 2: You owe $406.786456, your bill is $406.78, a balance of $0.006456 rolls over.
* Month 3: You owe $406.789684, your bill is $406.78, a balance of $0.009684 rolls over.
* Month 4: You owe $406.792912, your bill is $406.79, there's a $0.01 "other fee" line item on the bill, and a balance of $0.002912 rolls over.
It's unlikely that it would be illegal, or violate the contract with the payments gateway, if you charged the customer 406.78, and you made up the $0.003228 discrepancy so the gateway is paid off.
I wasn't talking about rounding up. I meant exactly the same thing you're saying about month 4.
> Because the bill included a detailed breakdown of all the fees by type and transaction.
Was it all rounded [down] to the nearest penny?
That type of bill can already fail to add up to the total very easily, like x.xx4 + x.xx4 + x.xx4. So I'm still not sure why there was a need to have a line item to explain this single penny.
Was there only a single charge on each bill that used fractional pennies? So that this was the only time that things wouldn't add up perfectly?
Canada abolished the penny some years ago. If some retail item in a store costs $2.03 and you insist on paying with cash, you will have to use a nickel and pay $2.05.
If the month's charges were 406.783228 and I get a bill for 406.79 then that seems perfectly good.
If I get a bill that says 406.78, plus a separate 0.01, that's weird.