Mission critical to email marketing is to have a Cart Abandonment Email Campaign. Your prospect has added a product to cart, you should first try and collect their email address. If they abandon the purchase process you can approach them in their email with an invitation to continue the purchase process. If this fail, you can offer a discount on their abandoned purchase to get them over the line.
If you have already spent money on paid advertising to get them to your site this is a highly cost-effective way to close the sale.