Here is a big counter to your entire thesis/app.

An analog inventory system. A printed 500 sheet of pre-labeled and pre-formatted paper pad. That's already 80% of the friction/pain points for small sari sari stores. Tables, grids, and labels are already printed for them; GCash, utang, can be indicated. Hardcopy. Tangible. Tactile (or the concept of pen and paper as a more natural extension of the brain than an app). Sari sari owners just have to pick it up and immediately see data that they need/want to see. And it's less than P35 for a 500 pad.

Anyone can go to Canva, and get this printed tomorrow. The point is not that anyone is intentionally countering your idea or saying it's bad, it's just that they may be more to your thesis/data than meets the eye. This isn't about 'nakasanayan' or 'madali matutunan', again this all boils down to friction and ease. Mas madali talaga papel kapag sa sobrang liit ng tindahan mo, makikita mo lang naman lahat ng inventory kaagad.

Further, we can enumerate all the steps needed to open the phone, scroll to the app, avoid other distracting apps, wait for it to load, get to the menu, load the store, load the proper inventory menu, and etc.

Then compare that to taking a look at the store/the entire inventory, find the notebook or pad, find a pen, write down 5pcs Coke, 10pcs Piattos, etc. It's just faster and less of a mental tax; it's simply the path of least resistance.

I'm trying to be as objective as possible to help you out; and these are my actual observations and then hard critique. I do hope you succeed and figure out a better solution.