Yes, you can pay later …. but you will have to pay the “late fee”…. that is to say that you’d be paying almost double the amount that you are required to pay right now.
The only thing that i can suggest is to solve the mock a week before the exam, because if you do that just 2 days prior to the exam date you wont have much time to work on the weak areas… Every person has his/her own strategy of studying… so the rest is totally up to you!