Don't keep selling once the deal is closed. This is simply a gracious gesture affirming how pleased you are to have them as customers.
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
I just wanted to drop you a short note to tell you how pleased we are to be doing business with you. Abc Company truly values its <customers/clients/etc.> and prides itself on providing quality <product or service>.
If you have any questions or would like any additional information, please feel free to reach me at <number(s)>. Thank you for the business and I look forward to working with you in the future.
encl: <List of enclosed items goes here>