Not only include all of the information about the invoice you are sending to your customer, take the time and have sales and marketing prepare one or two page product summaries easily folded in with the company invoice mailed to customers. Include all of the product lines that you offer that this current customer may not know about. Include a blurb in your software that adds a paragraph or two in the lower body of the invoice describing all of the skills you have to offer.
Include or insert a 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 not and reach a buyer at some time. It is worth your effort to market every time you have an opportunity.