In english saying "The prize is in the bag" means that you are virtually certain of winning it, before it has been awarded. Is this a similar idiom in italian? Or is it just saying that there is a prize inside the box?
No, just a prize in the box; the corresponding idioms in Italian are "l'abbiamo nel sacco" and "l'abbiamo in pugno".
just a small clarification
the correct idiom is "il premio ce l'abbiamo in tasca"
EDIT: grazie @f.formica
no, non è stato un refuso, quando l'ho scritto ne ero convinto :(
Ah, back to these sorts of sentences, are we, duolingo? "The prize is in the box", "the bee is in the sugar"... :P