Codeigniter is an excellent platform for building e-commerce applications. Speed and efficiency of the framework on the server makes a HUGE difference in e-commerce. Framework benchmark results matter - but for e-commerce these numbers are very critical. Especially if you are a small/medium business. Combine that with codeigniter's very easy to use caching - and a business can have a high performing website that can accommodate rushes of traffic and simultaneous users for peak Holiday shopping days etc.
I've used Codeigniter to integrate with payment processors, shipping services, amazon seller central, walmart, etc etc etc - and I have never had an issue with the framework. If there is development library for the API in PHP - drop it in the library folder - and it works. I'm developing an e-commerce integration with a large vendor - its using a library that requires namespaces and composer - we are running it on PHP 7 - its all working fine in codeigniter.
Besides looking at other frameworks, other choices would be - Open Source e-commerce applications. These can be stand alone like Magento, or built on top of a framework. Or SAAS like Shopify or Big Commerce. They are both rapidly adding more features and getting more developer API friendly.
