It is impossible for the writing company to guarantee the grade that will be produced by the writing of a hired writer. There are several factors that the writer does not know about and cannot control such as :
1. The grading considerations the teacher will be using
2. The current level of grades of performance of the student in the class which can affect the grade of the paper.
Then there is the matter of the student not actually knowing the writing level of the writer and his familiarity with the given topic. You will receive the paper that the skills of the writer can provide. You will also receive the paper that the writer thinks will get the best possible grade from your teacher. There are no certainties that would assure you of the grade you can receive, regardless of the assurance of the company or the cost of the paper once you are charged for it.