How to add a cart option

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i want a public app that allows to adds a service once per cart. Can't be a normal product (needs something else in the cart).
If the add-on is added to the cart, the information needs to accessed via API or send. Also needs to contain the option (checkox + adding the price to the cart for the service)

I looked into a couple of reference shops and some had an gift box option (it is no gift box, but Adding idea is similar). I also found multiple documentations, tutorials and other help.

Can somebody point me in the right direction, which way is future prove and not near EOL or bad practice?
The backend getting the information is written in PHP. I only need the information in case it is added and the checkout is finished.