There was a feature released around 8.1.3 to open URL in POS.
Follow the guide to add a button to POS via the Screen Layouts. In the Action, select Open URL.
Enter the PowerApps url. In this case I want it to open up as embedded rather than popping up with a new window.
Run your distribution job to have it show up on POS.
Below is an example I am working on. We have an external system managing licensing.
Just note, that it will pop up with a Microsoft login screen. You can tick the login to remember you.
If the screen doesn’t fit in the POS window. You can click on “Fit to screen” on the top right.
Hope this gives you ideas to take advantage of in Retail space. Possibilities are endless.