Newsletters should be considered a vital tool of ANY business. You've already done all the hard work of getting the visitor to your site! There have been multiple studies done over the years about the value of repeat business and how much cheaper it is to get a current customer (or previous customer) to repurchase merchandise.
Upselling your products to existing customers should be something done on a somewhat regular basis. We send out our notices each month of things that have happened, things to watch out for, and any specials that we have going on.
If you're not already marketing to your existing client base, you need to!