The shelf says three. The notebook says seven. The Instagram reply says it is available. All three were true on different days, and the customer who was promised the third one is now on the phone.
One number
Stock in is a delivery you counted. Stock out is a sale, a damage, or a use in the kitchen. If the counter sale and the online order both subtract from the same number, the page stops selling what the till just sold. Two notebooks, one for the shop and one for the counter, will drift by the end of the first week. That drift is the whole problem people call inventory.
You do not need a warehouse system to start. You need a count you believe for the twenty things that hurt when they run out, and an honest blank for everything else. A blank means you do not know. A zero means you counted zero. They are different, and a guessed zero hides a packet that is sitting in the back.
Variants, damage, and the quiet items
A red shirt in medium is not the same unit as a red shirt in large. If you count them as one product, the shelf will say you have stock on the day you only have the size nobody wants. Colours and sizes belong on the product, and each one has its own count.
Damaged goods are not a discount. They leave the count with a reason, or you will reorder what you already threw away and call it shrinkage. A tasting portion in a cafe is the same shape: it left the bag, and the bag's count should know.
Low stock is a line, not a feeling. When the oil tin hits the number you chose, the shop tells you before the customer does. The page should refuse the sale at zero rather than take the order and apologise. That refusal is the subject of never sell what you do not have.
The counter and the page are the same shelf
A kirana that sells on the counter in the morning and on the phone at night has one godown. Pasal keeps that count in one place, so a sale on either side spends it. The first setup is still a person standing in the aisle with a list, which is how to move the products across. After that, the list maintains itself every time somebody pays.