We quoted a payment integration in weeks. The code took exactly that. Then two months went to the client's compliance sign-off, a merchant domain nobody owned, and a bank contact who answered on Fridays.
So the estimate was right and useless. We now price the dependencies we don't control as explicit line items, with an owner's name next to each one. Half of our estimate meetings run on their side now, not ours.
You probably know who owns the slowest approval in your own delivery chain.