This is a formal acceptance of the job offer. There is no need to sell yourself at this point. Confirm your starting date so they know when to expect you.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I'm very excited about the prospect of working for <name of company> and am pleased to accept your offer of the position of <title of position>. As we discussed, I will plan to report to work on <starting date>.
I enjoyed everyone I met at <name of company> and look forward to being part of the organization. Thank you for the opportunity.
encl: <List of enclosed items goes here>