There are two ways to pay for the models, and you can mix them seat by seat.
Credit, on Our Keys
Credit is a dollar balance on your account. When a seat runs on Our Keys, each call is charged to it at the provider's own price plus 10% for serving it. A top-up keeps 4.5% for the product, and the payment fee comes out of it. Calls that we route through a model router also carry that router's fee, about five percent.
A new account starts with $2 of credit so the default mode works before you have set anything up. Pro includes $10 of credit a month and Team includes $20 per seat, pooled across the team, for the first three months; included credit is spent before purchased credit and expires at the end of the billing month. You can top up in Plan & Billing, and set auto-reload so a low balance refills itself. See "Included credit, top-ups and auto-reload".
Settings → Usage & Spend shows what was spent, by model, by device and by day.
Your own keys
Add a key from a provider and that seat bills your provider account directly. Credit is not touched, and there is no router fee. Choose this when you already have provider billing, want a model we do not serve on Our Keys, or would rather your prompts did not pass through our servers on the Mac.
Which to choose
Our Keys are the simplest: one bill, every model, no keys to manage. Your own keys cost less per call by the router fee and give you the provider's own usage reporting. Most people run the seats on Our Keys and add a key for the one vendor they already pay. On the free plan a seat on your own key, a custom endpoint, your company's cloud or a CLI works for one 14-day trial, then needs Pro; Our Keys and Free Key have no such limit.