The Invoicing Addon for G4PP (G4PPINV) allows you to interact with PayPal's RESTful web apis using OAuth 2.0 authentication. This article will take you through the steps for creating your first G4PPINV application.
With this first step you can go to https://developer.paypal.com/docs/integration/direct/make-your-first-call/ and get more details. Basically what you are doing is setting up an account that you can test and work with in PayPal's "Sandbox" where no real invoices, emails, etc will be sent.
When creating the App, I suggest naming it GreenTools for PayPal (G4PP) as this is the app we will be using.
After you have installed G4PP (and applied temporary keys for G4PPINV and GETURI if needed) you use the G4PPREGSVC command to register the invoicing service to your PayPal Sandbox account id. The command should look something like this:
There is no extra screen to go to. After this is done you should have completed the registration and the G4PP system will have received an OAuth 2.0 access token (as well as other information). Refreshing of the access token is done automatically when using the functions within the G4PP addons (such as G4PPINV).
A sample program named G4PPINVT1 can be found in the G4PP library in source physical file QRPGLESRC. Take your time and go though this program to get acquainted with all of the functions and options available with the G4PPINV addon. There are a LOT of them. And don't forget to change the account ID used in the program to your new ID created in step 1.
Once done and things work successfully, you should be able to log into your PayPal Sandbox account at www.sandbox.paypal.com and view the invoices you have created, canceled, refunded, etc.