In order to build good customer relations, it's often nice to send a brief cover letter along with the invoice. It gives you the opportunity to thank the customer for his business on a more personal level.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Thank you for giving Abc Company the opportunity to serve you.
Enclosed is our invoice for <specify what the invoice is for>.
Please let us know if you have any questions regarding this invoice. We look forward to serving you in the future.
encl: <List of enclosed items goes here>