I do have some users including myself as windows 10 UWP developer. I would like to allow my beta testers to test the In APP Purchases without being charged. is that possible?
Based on your description, you may want to publish your app in the store and only allow a few people to download and purchase IAP free to test it. My answer is yes.
Firstly, in your submission, on the pricing and availability page, choose hide this app and prevent acquisition. Customers with a direct link can see the app’s listing, but can only download the app if they have a promotional code and are using a Windows 10 device in the distribution and visibility section.
Then the app passes certification, generate promotional codes for the app and distribute to your testers.
Finally, you could need to set IAP free when you submit Add-on in the Store.
You can refer to beta testing and targeted distribution for more details.