Option 1
After clicking Finish Your Order button, you can see your total Amount Credit showed on the below of your order Total Due in a new page:
If your Amount Credit is less than the amount Total Due, all your credit under your account can be transferred to the order and you can go ahead to pay for rest payment continue via paypal or other payment methods.
Option 2
If you intend to pay for your previous order, you can open My Account page just click Pay Now button as the below picture:
And then you can see your total Amount Credit below your order payment instructions. (The Steps is the same with Option 1)
Attention of Account Credit Using:
You will see the Account Credit amount in your Finance Manage page as below, if your account balance is positive, that means you have credit in your account which you can use it to pay for your current order:
You also can check recorder how and when you used the credit in here, and New Credit will should the credit amount still left of your account, everything is clear.