We looked through thousands of eCommerce APIs and put together a list of the most popular & cheapest ones available.

Product Advertising API gives access to a lot of the data used by Amazon including the items for sale, customer reviews, seller reviews, etc

Product Advertising API

Amazon has developed a world-class web service that millions of customers use every day. As a developer, you can build Product Advertising API applications that leverage this robust, scalable, and reliable technology. You get access to a lot of the data used by Amazon including the items for sale, customer reviews, seller reviews, as well as most of the functionality you see on Amazon, such as finding items, displaying customer reviews, and product promotions. Product Advertising API operations open the doors to Amazon's databases so that you can take advantage of Amazon's sophisticated e-commerce data and functionality. Build your own web store to sell Amazon items or your own items.

Best of all, Product Advertising API is free. By signing up to become a Product Advertising API developer, you join the tens of thousands of developers who are already realizing financial gains by creating Product Advertising API-driven applications and web stores.

 What's new in Product Advertising API 5.0
On a high level, Product Advertising API 5.0 offers following new features:

- **Ease of integration:** Out of the box SDKs in popular languages for easy integration and adoption. The SDKs takes away all the heavy lifting of signing the request, request serialization and response de-serialization.
- **Lighter, Slimmer and Faster**: PA-API 5.0 is built from ground-up to offer faster response times and uses Resources over ResponseGroups used earlier. PA-API 5.0 supports the lighter and slimmer JSON format over XML supported earlier.
- **Better Customer Experience**: PA-API 5.0 offers features consistent with Amazon retail like LanguageOfPreference compatibility, PrimeExclusive eligibility, Multiple and intuitive Sales rank information in context of a particular BrowseNode, etc.

Product Advertising API

Integrate eSignatures into your application in minutes.

 eSignature API overview
Integrate eSignatures into your application in minutes. DocuSign's secure and award-winning eSignature API makes requesting signatures, automating forms, and tracking documents directly from your app easy.

 Features
- **Award-winning eSignature API**
Multi-award winning platform including winner of the 2021 Best Business Software API by API World.

- **Provisioned for growth**
Handling everything from proofs of concept to worldwide enterprise integrations at scale

- **Robust and flexible**
Offering more than 400 endpoints for more than just signing: manage, collaborate, act on agreements, and more

eSignature API 

The PayPal REST API is organized around transaction workflows, including: orders, payments, subscriptions, invoicing, and disputes.

The PayPal REST API is organized around transaction workflows, including: 
- orders, payments 
- subscriptions, invoicing
-  disputes

The API uses standard verbs and returns HTTP response codes and JSON-encoded responses.

To get started with the PayPal REST API, first create a developer account on the Developer Dashboard. From there you can generate your credentials, authentication token and sandbox accounts.

Logging into the Developer Dashboard to get credentials and create sandbox accounts requires a developer, personal, or business account. Each account provides different levels of access to PayPal functionality. If you need to access a capability you don't currently have, you can upgrade your account on the My Account page in the Developer Dashboard.

Depending on your **account type**, you can enjoy the following features:

- Access sandbox
- Send and receive money
- Go live

Paypal Rest API

A REST API that extends support for inventory, sales orders, and shop management on the Etsy platform.

Welcome to the improved **Etsy Open API v3**, a REST API that extends support for inventory, sales orders, and shop management on the Etsy platform. These guides support current and future app developers as they build tools to integrate with and automate processes for Etsy shops and customers.

By default, all new applications support essential access, which is authenticated read/write access to a shop granted by the owner and controlled by Oauth token scopes. This supports designing access controls for different application users into the app, such as reading data on receipts and billing or creating, editing, and deleting your shop's listings.

Etsy Open API v3

The WordPress REST API provides an interface for applications to interact with your WordPress site by sending and receiving data as JSON

The WordPress REST API provides an interface for applications to interact with your WordPress site by sending and receiving data as JSON (JavaScript Object Notation) objects. It is the foundation of the WordPress Block Editor, and can likewise enable your theme, plugin or custom application to present new, powerful interfaces for managing and publishing your site content.

Using the WordPress REST API you can create a plugin to provide an entirely new admin experiences for WordPress, build a brand new interactive front-end experience, or bring your WordPress content into completely separate applications.

The REST API is a developer-oriented feature of WordPress. It provides data access to the content of your site, and implements the same authentication restrictions — content that is public on your site is generally publicly accessible via the REST API, while private content, password-protected content, internal users, custom post types, and metadata is only available with authentication or if you specifically set it to be so. If you are not a developer, the most important thing to understand about the API is that it enables the block editor and modern plugin interfaces without compromising the security or privacy of your site.

Wordpress REST API

An API that lets you programmatically manage your Shopping product items.

You can use the Content API for Shopping to automate Google Merchant Center account management.

Here are some things you can do with the Content API for Shopping:

- Upload products
- Manage inventory
- Manage orders and returns
- Manage Merchant Center accounts
- Link your inventory to Google Ads

After uploading products, you can take advantage of other Google for Retail solutions, like promoting your products in Shopping campaigns or selling your products through Buy on Google.

 Content API for Shopping

Shopify Checkout API allows building a sales channel with lower integration effort.

Building a sales channel with the REST Admin API's Checkout resource or Storefront API's Checkout object represents lower integration effort because Shopify makes it easy to handle the complexities of payment processing.

- Shopify offers a variety of payment processing options, depending on your business case and the needs of your app.
-- Learn about payment processing with the REST Admin API.
- Shopify handles paying out to the merchant for you.
- Shopify keeps track of orders and fulfilments.
- Shopify handles disputes, refunds or cancellations.

If you want to direct customers who want to buy a specific product from the sales channel directly to a merchant's store checkout, you can build a sales channel with cart permalinks.

The Top 50 Ecommerce Apis for Developers