Can I ask for a help? On how to program billing system with source code? I just need it for my case study.

Everyone starts somewhere so what you need to know is that OPEN SOURCE for so many systems is out there. I can assure you that I don't know them all but will research such on Google. Here's my google on that and sure enough, there's the source code.

https://www.google.com/search?q=OPEN+SOURCE+BILLING+SOFTWARE+PYTHON

I added the word PYTHON since you must be very new to how to find such and PYTHON is not that hard on new folk.