The cheapest way to get more customers is to upsell and cross-sell to existing customers. And the cheapest way to keep in touch with existing customers and inform them about your new offers is by using Email Marketing.
The easier you make it for them, the more likely they are to do it.
With email marketing, you can send your offers straight to the inbox of your potential clients or existing customers and with a single click of a mouse, they would be on your website or eCommerce shop adding the item to their shopping cart. And then within a few more clicks, they would have finished purchasing the item.
You can monitor the number of people opening those emails, clicking on a button and also, forwarding them on.