A job acceptance letter is a formal way to show your consent for a job offer. However, it must not be done in a casual way.
It involves a thorough review of the salary and benefits offered by the employer. After you are fully satisfied with the compensation package, only then should you send an acceptance letter to the employer. A hastily-made decision because you are too excited to receive an offer could put you in a position you regret later.
via How To Write A Job Acceptance Letter That Will Impress | CAREEREALISM.