Dear Ms. Jones:
I am pleased to inform you that I accept your offer of $____ per (month/week/hour) to work as a mechanical engineering co-op at XYZ Corporation. I will be available to report to work on Monday, August XX, 20XX. Please let me know if I need to take further steps to finalize our plans.
I appreciate having the opportunity to join your team of student employees and look forward to a successful [fall] co-op assignment.
Dear Mr. Jones:
Thank you for offering me the opportunity to co-op with [company name]. I appreciated the chance to meet with you and to learn more about your company.
Now that I have gone through the interview process, I have been able to clarify my career goals. Based on what I have learned about the various employers, I believe [company name] is not the best match for my skills and interests at this time. Please remove me from consideration for your current opportunity.
I appreciate your time and the information you have shared. Best wishes as you seek the best co-op for your needs.