When your shopping portal integrates with websites like UPS, FedEx and USPS, you can offer this info to the visitors quickly and thus manage your margins a lot on shipping costs. The shipping costs are catered via the API integration.
Adding features that can improve upon the functionality of your eCommerce apps can help in getting your target audience to explore a number of products you sell online.
There are a few popular responsive web design frameworks, which are popular among the web designers and developers, which are promise a number of features. These have their own strengths and weakness and thus can be fit to certain areas of applications.