Don’t you know PrestaShop (PS)?
Take a look here 🙂
It’s a modular system that let you create a rich e-commerce platform by supporting a vast number of different features. These features are implemented via modules that hooks right into the core of PS.
There are a lot of modules, free and not, and the community is pretty active. Some of them are released together with PS and are directly developed by the PS group.
However some of them are limited (e.g. Cash On Delivery) and so…developers to the rescue!
There are a lot of custom modules posted in the PS dedicated forum section but actually you can also directly fork PS from GitHub, develop your changes to the core modules, and then ask a pull request.
Here’s an example of the over mentioned Cash On Delivery module enriched with the possibility to add a custom fee.
I’ve added this feature too and published the modified version of the module on GitHub. Actually I didn’t fork the entire PS repo but I’ve just created one with only my module.