That's great marketing advice, Pete. A sale is never completed. It's only part of the process on the marketing road to growth.
Always put a flyer, product list or something in with the product packaging. Also, make sure you invite them onto your mailing list or emailing list and make sure you use that list for announcements, special offers or anything you can dream up to keep in touch every two or three months.
It costs a lot of money to get a new customer relative to reselling to an existing one. The existing one has paid you money and knows they got something in return. That is, trust established. Don't waste it.