Log into your account, click “My Credits” on left side. You can see Credit Information and Transaction History as below picture
You can see how much credit you have in shopping cart. If you want to use all credit, select “Use maximum credit” then click on “apply credit”.
For example, you have 41.89 euros credit available (as in below picture), but you only want to use 10 euros this time, you can just input 10 in the blank and click “apply credit”.