Store credits and promo codes can be used with your future purchases, but cannot be applied to previous purchases.
Simply add your desired models to your cart and go to https://www.shapeways.com/cart. Or click the "cart" icon at the top right corner of the screen, and click either "Edit Cart" or "Checkout Now."
Here you can apply the credits or the promo code on the right side of the page, your total will be recalculated as soon as you click the white "Apply Credit" button:
To check your store credit balance, please go to http://www.shapeways.com/credits
You can also access your credits by logging into your account and clicking your profile photo at the top right corner of the screen, then clicking "settings" when the menu appears. Next click "Store Credit" at the top of the screen to access all of your available credits.