The same problem kept arriving at the desk.
Shop owners keep honest books — in a notebook, on the back of a bill book, in their head. Then, every month, someone loses a day turning those books into something GST-shaped, chasing who paid what, and finding the purchase bill that went missing.
Khata Simple is the book they already keep, made to add itself up. A bill raised at the counter is already in the sales register, the customer's khata and the GST summary. A payment received settles the right bills. Nothing has to be entered twice, and nothing needs an accountant's vocabulary to use.