The body of your new customer welcome letter should include an introduction to your company, as well as reiteration of your capabilities.
Here is an example of a company introduction:
"At Smith Consulting, we pride ourselves on offering our customers responsive, competent and excellent service. Our customers are the most important part of our business, and we work tirelessly to ensure your complete satisfaction, now and for as long as you are a customer."
In this section, you will also want to introduce yourself, and let the customer know what role you will play in his or her dealings with your company.
Here is an example of a personal introduction:
"I am also happy to inform you that I will be your primary point of contact at the company, and I encourage you to contact me at any time with your questions, comments and feedback."
View the full-size version of the new customer welcome letter example shown here.