What is the most annoying part of billing software for your shop?

Hi, I'm Madhan, the maker of Invoiceo, a free offline billing app for small shops. Before our launch I would love to learn from people who run a shop or do the billing:

1. What do you use today (paper, Excel, Vyapar, Tally, something else)?

2. What is the most annoying part of it?

3. What one feature would make you switch?

Your answers will decide what we build next. Thank you!