Keeping In Touch
There are a number of options the system offers you for keeping in touch with your customers, potentially in combination with external items.
First, consider collecting customer email addresses from those willing to give them. One thing that can incentivize this for your customer is to utilize the email receipts feature. You can include coupons with the email receipts - this is set in the same place as the email receipt option, under tools -> options -> internet features.
Once you have those email addresses, if you get permission to email your customers with a newsletter, you can use a service like Constant Contact to email out regular communications. These can keep customers informed of events, new products, and other goings on in your store.
Second, you can use ExtremePOS with a label printer to do physical mailings - with Bixolon and Zebra label printers, if you print a label for the customer but do not include a barcode it will be formatted in a mailing style. You can print these for a particular customer, or for all of the customers you've had set up.