Formal:
* Consider it settled.
* Rest assured.
* You can count on it.
* It's a sure thing.
* It's a done deal.
* It's a certainty.
Informal:
* You betcha.
* You got it.
* No doubt about it.
* Absolutely.
* It's a lock.
* It's in the bag.
The best synonym to use depends on the context and tone of your conversation.