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Not only include all of the information about the invoice you are sending to your customer, include a comprehensive list of the product lines that you offer that your current customer may not know about. Program a text box or little blurb in your software that adds a paragraph or two in the lower body of the invoice or second information page describing all of the skills you have to offer.
If you prefer, include or insert a separate flyer if space allows indicating all of the services or products you sell (short and brief bullets). It is free and at worst will be tossed, but it also may be forwarded to the right buyer(s) sometimes. It is worth your effort to market every time you have an opportunity with every piece of mail or every email you forward.
