The UI buttons are so small they make it too hard for me to consistently click on them, especially when I have to focus on the customers as well.
I would suggest to add keyboard inputs as alternative controls to add products (with num pad or QWE/ASD/ZXC) and switch customer (with arrow keys), or at least makes all the buttons bigger and their detection boxes squares or circles. I would also suggest using left mouse button and right mouse button for adding and deducting products instead of having to click on 2 smaller buttons.
Furthermore, I wish I could switch to the records of a customer just by clicking on them instead of scrolling through the pages, but I can see it as an intentional design and part of the challenge.
Really love the ASMR sound design, though.