You can buy anything in the shop with your credit, doesn't matter what it is. If what you want is more than your credit, you just pay the difference. If it's less, the credit stays on your account for future use.
Just make sure that the credit is correctly allocated to your account.
Once I traded in a few games (it amounted to roughly R600 store credit) and when I went back later that week the credit was not on my account because the douche behind the counter allocated it to someone else's account!