How do you say you have under-billed a customer

Question

Lets say i'm billed a customer for services. But realized i didn't charge them the correct amount. So now i'm going back and telling them.Oh i have under billed you so i need to charge you the correct amount.

Answer 1

We don't really have many nifty constructions like "underbill" in Spanish. Instead you'd have to just say that there is an error or an insufficient charge in the bill. Some examples:

"Disculpe, hay un error en la factura", or "Mil perdones, marqué la cantidad incorrecta en la factura".

While I am personally not comfortable saying this, because it is always kind of awkward and ugly to have to run after someone to tell them that they have to pay more, a good approximation to what you stated in your question could be:

"Mis disculpas, coloqué un cargo insuficiente en su factura. Por favor permítame hacer el ajuste."