Thanks to Jean-Luc Arnaud, we got a sample project to show how to use Paypal in a Xojo application.
The application opens a window to run the express checkout for purchasing something, e.g. the app license and than checks output. This way you can see if payment succeeded or failed.
For that we use a HTMLViewer to load the paypal website and in background a query using CURL Plugin to initiate the payment.
PS: Will be included in next prerelease.