What it does / Organisation
API access
One key per application, and per key what it may do: read customers, change customers, read projects, submit a request. Alongside it you set which web address it may call from and how often per hour. If something runs away with itself or a key gets out, you withdraw that one key and the rest keeps running.
For somebody else's application there is OAuth 2.1: the user signs in at Oifis itself, sees what is being asked for and gives permission — your password never passes through it, and what you allowed you withdraw later from one screen. The description of the way in is published in OpenAPI format, so your tooling can read it in rather than somebody typing it over.
What it costs
Or add it on its own, for € 15 / month
Keys and OAuth 2.1 for whatever you build around it or have built, with the OpenAPI description alongside. Per environment, however many keys you issue.
You get an environment of your own with an example company in it: colleagues, customers, projects, hours written and invoices at every stage. Nothing to set up, just look around.