Book excerpt: Welcome to "Practical PayPal Integration in ASP.NET Core". This book will provide useful resources on PayPal Checkout and Subscriptions APIs and SDKs, as well as their applications in ASP.NET Core projects. I hope that this book will be useful for .NET programmers, software developers, and students of all skill levels, who are interested in integrating the PayPal REST APIs into their ASP.NET applications. You may often get confused with PayPal because there are so many frameworks and options to do the same thing. In fact, there are over 50 PayPal services, APIs, and SDKs out there – you just do not know which one is suitable for your applications. In this book, I will choose the latest PayPal products, services, APIs, and SDKs, and integrate them into your ASP.NET Core applications. Here are PayPal technologies that I plan to use: •PayPal Smart Payment Buttons – These new buttons, introduced in June 2018, provide a simplified and secure check out experience. They intelligently present the most relevant payment types to your shoppers automatically. •PayPal JavaScript SDK – This SDK integrates Smart Payment Buttons into your web site and renders them to a container DOM element. •PayPal Checkout .NET SDK – To simplify integrations, PayPal provides a .NET SDK for their Version 2 REST Payment API. Using the SDK over a direct integration allows the SDK to handle authentication on the server side for you. •PayPal Subscriptions API – I will implement a Subscriptions .NET SDK based on PayPal Version 1 Subscriptions REST API. This SDK allows you to easily integrate PayPal Subscriptions into .NET applications. In the first part of this book, I will explain how to use the PayPal Checkout .NET SDK to get start with the PayPal Version 2 Payments and Orders REST APIs. I will use various examples to demonstrate the procedures to complete a PayPal checkout transaction, including how to set up the development environment, how to integrate and render Smart Payment Buttons, how to create and manage orders, and how to capture and verify the transaction. In the second part of this book, I will switch to PayPal Subscriptions API. This API has not been ported to Version 2 yet, so we have to use Version 1 API for subscription. To make matters even worse, PayPal has never released a stable Subscriptions .NET SDK, which makes it difficult for .NET developers to integrate PayPal Subscriptions API into .NET and .NET Core applications. Therefore, I decide to implement a simple PayPal Subscriptions .NET SDK, which simply exposes various static methods that can be used to integrate PayPal Subscriptions API into .NET applications. I will use several examples to illustrate how to use this SDK to complete subscription transactions.