This is what i would have told them:
Take .002 cents, Multiply it by 500, what do you get?
1 cent.
1 cent right?
Let's do this, 35,000/ 500 (Divide 35 Thousand, by 500, to see how many cents we get)
Oh, 70 cents? Makes sense... but wait a sec... you charged me 70 dollars?
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Lol ya i know guys i used nice numbers.
His number was like near 35 thousand, so i just used that, but you get the idea, and he owes like 77 cents....
Maybe they would have gotten it that way?
Possibly?
Anyways, that's just how i would have explained it.

~~Datruth