E-commerce covers selling online: the storefront, the catalogue, checkout, payments, and the operational tail of orders, stock, returns and tax. Providers in this category range from firms that configure an off-the-shelf platform to teams that build bespoke commerce systems, and the two are priced and maintained very differently.
The decision that shapes everything else is platform versus custom. A hosted platform is faster and cheaper to start and constrains what you can change; a custom build inverts both. Beyond that, ask where the catalogue and stock actually live — a storefront that keeps its own copy of stock, separate from whatever your business already runs on, is the source of most of the operational pain in this category.