In the real world there are e-commerce service providers who present some degree of service products to the e-commerce web site. Some offer the turnkey (everything you need) service and some offer components like carts. PayPal is the ex of e-commerce service provider. While they don't offer web sites and hosting (maybe just around corner) they do offer cart hosting. Every service provider presents a different take on giving their service and various interfaces. UPS is another ex of a service provider.
Determine the e-commerce service provider online and examine their offerings along with the detailed view of how it may work and integrate with the functioning e-commerce web site. Focus on providers of e-commerce turnkey products or sites having e-commerce tools.