We are a specialized outsourced accounting service/our company is an outsourced accounting firm
(could also be "finance and accounting outsourced service"). We offer customized services to customer across different business sectors, such as service, commerce and industry.
You can find other ways to say that, and it may not apply to every firm (cover all bases), but it´s a kick-off. Certainly other comments will come to improve my answer. By the way, it´s a student point of view here, so not meant to be for professional use. Sometimes we have to acknowlege our (very) limited skills on some field.
Case in point.