We have two types of ecommerce solution. The first, WooCommerce (great name) is a bolt on system to a standard WordPress website. WooCommerce is a fantastic option for people that only have a small number of products to promote. As a bolt on, it can be added to your normal website so you can keep all your content but include product or service sales.
Once the shop is built there is no ongoing cost other than your payment providers fees. WooCommerce is ideal for new start ups, testing the water. Established businesses that want to include a few products or for service related businesses who would like to be able to sell service packages and take online payments.