An order arrives at 19:40. At 08:15 the next morning someone opens the webshop, reads it, types the name and address into the accounting package, then copies that same address onto a list for the driver. Twelve facts, three times, by hand.

Nobody chose this. Each tool arrived on its own day to solve its own problem: the till first, then the webshop, then the accounting package your bookkeeper asked for, then the booking form. They all work fine. They just do not talk to each other, and the gap between them is filled by a person with a keyboard.

Where the double entry actually hides

It is rarely one big moment. It is a trail of small ones, and the people doing it have stopped noticing.

In a shop: the order, the customer record, the stock count, the invoice. In a garage: the parts order, the job card, the workshop planning, the invoice. In a clinic: the appointment, the patient file, the reminder, the billing. In a small agency: the proposal, the project, the hours, the invoice.

The tell is simple. If you can name a fact — a phone number, an amount, a date — that a human types into more than one screen in the same week, there is a link waiting to be made.

Second tell: the correction. Someone changes a delivery address in one system and now has to remember the other two. Half the time they do. The other half is where your wrong deliveries and your “we never received that invoice” calls come from.

What a connection is, in one line

An API is a door in a program that another program is allowed to open. Instead of a person reading one screen and retyping into another, one system asks the other for the data directly and gets a clean answer.

In practice there are three levels, cheapest first.

A scheduled file. One system writes a file every night, the other reads it in. Unglamorous, cheap, and completely fine for anything that does not need to be live — the day's takings into the bookkeeping, for instance.

A ready-made connector. Your webshop and your accounting package may already have an official link, built and maintained by one of them, often included or a few tens of euros a month. Always check this before you pay anyone to build anything. In our experience it covers more situations than owners expect.

A custom link. Written for your situation, because one side is old or your process genuinely is not standard. It costs more up front, and someone has to own it afterwards.

Which link to build first

Do not connect everything. Connect one thing, well.

Take the arithmetic seriously for a minute. Twenty orders a day at three minutes of retyping is an hour a day — roughly five hours a week, every week, at exactly the hour your staff should be doing something else. Then add the error cost: one wrong address is a second delivery run, an apology and a discount.

Rank your flows by volume times irritation, take the top one, and leave the rest alone until that one has run quietly for a month.

Decide who is the boss of each fact

This is the part that goes wrong, and it is not technical.

Before anything is built, decide per fact which system is right. The webshop owns stock levels. The accounting package owns invoice numbers. The booking tool owns appointment times. Write it on one sheet of paper.

Then ask the boring questions. Does the link run one way or both ways? One way is simpler and safer for most small businesses. What happens when it breaks at two in the morning — does anyone find out, or do you discover it three weeks later in the bookkeeping? Can a human still override a record by hand when a customer phones? And who adjusts things when one of the two suppliers changes something?

A link without an owner degrades quietly. That is the real failure mode, not the code.

What to do this week

  • Put a sheet of paper by the desk for five days. Everyone writes down each fact they type into a second screen. You will have your list by Friday.
  • Take the heaviest flow and email both suppliers one question: “is there a standard connection between your product and X?” Surprisingly often the answer is yes, and you are already paying for it.
  • Write the source-of-truth sheet: per fact, one system that is right when they disagree. Ten minutes, and every future link leans on it.

If you would rather have someone look at your two systems and say plainly whether a standard connector will do or something really needs building, that is a conversation we at ITSysPro are happy to have, with no obligation.