modular

Spree is a complete, modular & API-driven open source e-commerce solution for Ruby on Rails, offering a flexible platform for building sophisticated online stores.

Solidus is a free, open-source ecommerce platform built with Ruby on Rails that gives you complete control over your store with a focus on flexibility and customization.

Thelia is an open source and flexible e-commerce solution based on Symfony components, offering a modern approach to building online stores with a focus on modularity.

Para is a flexible and modular backend framework/server for object persistence, API development and authentication. Built with Java, it provides a comprehensive backend-as-a-service platform with built-in database management, user authentication, and API generation capabilities.

Drupal Commerce is a popular e-commerce module for Drupal CMS, with support for dozens of payment, shipping, and shopping related modules, providing a flexible commerce solution.