winning submission

18:03 May 7, 2015 

I agree that the text is not the best, but I would be very careful making assumptions as to the number of winners. Yes, they use prizes in plural, but it is entirely possible that one winner gets multiple prizes. I have seen contests where the winner received a scholarship, plus a paid trip, plus a package of goods from the sponsors.
As to: "the participant will be informed of the exact details of the winning submission", this is definitely something that needs to be clarified with the client. My guess is that it is not the details of the submission that the winner will be let know, but either the prize he/she won, or the steps he/she needs to take to claim the prize, for example sign a release so that the company could use the submitted materials, agree to participate in some promotional events, etc. Either way, ask the client, that is the safe approach.

09:10 May 7, 2015 

I think that the text needs some correction:

After determination of the prizes, the respective winners will be notified by xxx according to the specified contact details.
In the course of this notification, each participant will be informed of the exact details of the winning submission.