In my opinion, it's more commonplace to have the situation the other way around.
I have had clients come to me asking if I could work on a model paper for them but, they make it clear that I should only provide X amount of information in the almost finalized form of the paper because they want to add their own touch to the research paper or essay.
By leaving the paper merely completed instead of totally completed, the client has the chance to add some information taken from class lectures and other sources such as their class reading material.
Any company that promises any grade is a)lying and b)most likely a scam.
No one can guarantee a grade except for the person actually grading the paper; the best writer/researcher in the world (if such a thing could be objectively measured) could turn in a paper that a professor decides to give a "B" for some reason.