App or API: which one is yours
The WhatsApp Business app installs on a phone, is free, and gives you a business profile, hours, quick replies, labels and an away message. If you answer personally and the volume is manageable, you need nothing else and should not pay for anything else.
The API is a different animal: it is not an app but a connection between your system and WhatsApp. It is what you need for a booking made on your site to trigger a confirmation with nobody typing it, for several people to handle the same number, and for everything to be logged.
The signal to move is usually one of two: more than a couple of dozen conversations a day, or more than one person answering the same number and tripping over each other.
What it costs
The API has no WhatsApp line rental: Meta charges per conversation started, at different prices by conversation type and country. Service messages inside a conversation the customer opened are usually treated differently from ones you start with a template.
For a small business that normally lands at a small monthly figure, but the sum has to be done against your real volume before building anything, and it should be clear that the money goes to Meta directly. If a supplier resells it with a margin and will not show you the tariff, ask why.
What to automate and what not to
What works, and what nobody complains about receiving: immediate confirmation of a booking or order, a reminder the day before with a cancellation link, notice that the order is ready or the engineer is on the way, and an out-of-hours message saying when you answer.
What does not: bulk promotions, "we miss you" messages, anything arriving without the person having done something. This is not only a question of taste. Sending commercial communications by electronic means in Spain generally requires prior authorisation, and your customers’ numbers are personal data with everything that entails.
The practical rule we use: if the message responds to something the person just did, automate it. If the message exists because you want to sell today, write it yourself, and ask first whether you should.