The way to be polite in English is to be tentative. Even if there's a glaringly obvious mistake, you would still draw attention to this by saying.. There appears to be an mistake. There seems to be an mistake.. You could soften this further using terms such as 'inconsistency' 'miscalculation' or 'omission' if appropriate, as an alternative to 'mistake'. Note that in general English-speaking cultures tend to be more polite and indirect than Slavic cultures do. What counts as a normal statement in Slavic cultures may appear direct to the point of rudeness in the eyes of a British or American person. So, if in doubt, always be as polite as possible!

Politeness depends mostly on the meaning of what you say, not on the choice of words. Here, the best you can do is to make it a question, and not an accusation. But you have to be direct and clear. Paul's suggestion is good. "Is there a mistake on invoice?" is another possibility. Of course, you then need to be a clear as possible about what the mistake is. "Mistake" is a good word here because it is clear and direct, but it doesn't suggest that they were trying to cheat you. Everyone makes mistakes. Probably the essence of politeness here is to provide the right information to make it easy for them to solve the problem. For example, enclose a copy of the invoice and circle the specific item or items that are in error. For example... just imagining a situation... circle the place where the invoice says "net 14 days" and then attach a copy of a letter they sent you you earlier circle the place where it says "We will be glad to do business with you on a net 60 day basis..." that kind of thing. Do as much of their work for them as you can. It is not pleasant to hear "I think you made a mistake," but there are no magic word choices that can make it pleasant.

"I believe there is a mistake in the invoice." Then give details