Off-the-shelf software is, for all intents and purposes, aimed at solving fairly simple commerce transactions. A sample case would be having users browse for products, add them to a cart, and then check out. For scenarios such as this, there are many solutions available, ranging from WooCommerce powered by WordPress to Magento, which is a largely popular commerce platform.
However, what about when things become more complicated? What if your products require sophisticated calculations or customization? Or if you have extraneous transactional calculations? In these cases, a customized e-commerce platform may make sense for your organization.