"There is a mistake in the bill."
Translation:Estas eraro en la kalkulo.
I agree I don't like kalkulo in this context either, especially since the course just got done beating me over the head with "Mi mise kalkulis, do mi miskalkulis" and "Miskalkulo: unu kaj tri estas ok" leaving little doubt that "Kalkulo" is a mathematical calculation. It's use as "Bill" here is very confusing and ill worded. However as a novice speaker I really couldn't come up with an alternate except maybe Acxetajxo? What do you think?