When you promise 48 hours in writing, three things have to be true. You need genuine slack in the bench, not a schedule that is 100% allocated. You need a brief-acceptance step, because a vague brief makes the clock meaningless. And you need to eat the cost when you miss, or the promise is decoration.
We hold roughly 15% of the bench unallocated to make the first one true. That is a real cost and it shows up in our rates. What you get for it is the ability to quote a client a date on the sales call instead of saying "let me check with my team."
The free-if-late clause is the part vendors resist hardest, because it converts a soft promise into a liability. That is exactly why it belongs in the agreement — a commitment with no consequence is a preference.