Both APIs (Application Programming Interface) and SDKs (Software Development Kit) support the development of a webpage. If you have created a website with all the front-end and back-end functions, API is the required software connection to launch it on the internet and connect it to other websites.
On the other hand, if you want to make changes or upgrade your existing webpage by adding or removing any features, you require SDK toolkits to make the relevant changes. Though you can still code and make modifications from scratch, SDK kits reduce the work as they already have all the commands, protocols, libraries, and compilers installed for instant use.
Generally, an e-commerce platform or website hosting platforms like amazon or Facebook provides both the elements, which other developers can openly use to build and connect their sites, respectively. For example, if your website is already developed but now wants to add direct communication channels, you can program email or telephone call connectivity.
But you can effectively use Amazon or Google's email SDK to add that feature. However, if you have created a new platform and want to launch it, you have to use API to connect it to platforms like Shopify, Magneto, or Amazon. Visit here to know more - https://api2cart.com/api-technology/api-of-ecommerce-platform/