The price book, and why apps gave up on it

If you grew up in a house where money was watched, there may have been a notebook in a kitchen drawer with prices in it. Peanut butter, the big jar, four dollars and something at one store and a little less at the other one across town. That notebook is a price book. It is one of the oldest tools a household has for spending less, and it works as well today as it ever did. The trouble is that almost everyone who starts one gives up within a month, and we think there is one reason for that.

What a price book knows that a store sign does not

A shelf label tells you what a thing costs today, in this store. It cannot tell you whether that is a good price. For that you need to know what you paid last month, and what you paid at the other store, and whether the pack quietly got smaller since spring. Only a record can hold those three things together.

Once you have a few months written down, the questions that used to be guesses start having answers. What did I pay for this last time? Where have I paid least for it? Is the price in front of me high for this particular store? A household that can answer those three questions in the aisle spends less without trying very hard, because the decision is made with information instead of a vague feeling that things have gone up.

Why the notebook goes quiet

We have kept a few price books over the years, on paper and in spreadsheets, and every one of them died the same way. The recording happened at home, after the trip, from a pile of receipts on the counter. That is a second job. It lands on the evening when the shopping has already eaten the afternoon, the bags are unpacked, and the last thing anybody wants is to sit down with a stack of thermal paper and a pen.

You skip one week because you are tired. The next week there are two receipts to enter, which is worse, and it is easier to skip again. Within a month the book has a gap in it, and a book with a gap feels broken, and a broken book gets abandoned. It is not a failure of willpower. It is a job placed at the worst possible moment of the week.

What the apps inherited

The first generation of price book apps copied the notebook exactly, including its flaw. They gave you a tidy place to type prices into, and they still expected you to do the typing at home. Some of them later added receipt scanning, which helps. A photograph of a receipt is quicker than typing it, and the app fills in most of the lines for you. Even so, it is a session. It is a thing you have to remember to do after the trip, and anything you have to remember to do after the trip is a thing you will eventually stop doing.

The moment the price is easy to record

Here is what we noticed when we started paying attention to our own habits. There is exactly one moment when writing a price down costs almost nothing, and it is the moment you are standing in front of the shelf with the thing in your hand. The price is right there on the label. You are already reading it, because you are deciding whether to buy the thing. Typing four digits into your phone at that moment takes about as long as reading them.

That is the moment ManyStops is built around. Each store's list comes up in the order you walk it, with what you paid for each thing here last time. You type the price on the shelf if you like, and scan the barcode. The item is in the cart and the price is in the book, and you never think about it again. Anything you skipped, the receipt at the end fills in. There is no session afterwards, because the shopping was the session.

What to write down

Whatever you use for a price book, four things are worth recording, and the order matters less than the habit.

• The price of one. Write down what a single item costs, the way the shelf shows it. Three jars for five dollars is one dollar sixty-seven each, and the total works itself out from there.
• How many you bought. Two jars at one price is a different fact from one jar at twice the price, and it matters when you look back.
• The size. Twelve ounces or eighteen. Two jars of different sizes can only be compared by the ounce, and the size on the label is the only way to get there.
• The brand. A store brand and a name brand are two products with two prices. Averaging them together tells you nothing about either.

What you get back

After a few trips the book starts talking. The next time you stand in front of the peanut butter it shows what you paid for it here last month. If you have paid less for it at another store you already go to, it says so, with both prices side by side. If the price in front of you is well above what you usually pay here, it says that too, and you can put the jar back and wait.

None of that comes from a catalogue, or from a company that scraped prices off the internet, or from other people's guesses about what your store charges. It comes from what you paid, in that store, on that day, because you were standing in it. That is the whole point of a price book. It only ever needed to stop being homework.

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