At checkout the option to pay using your cash credit balance instead of using your payment card should appear as long as the amount of cash credit on your account is equal to or greater than the cost of what you're purchasing.
If it's less than that then the option to (part) pay using cash credit will not appear.
If the option should be appearing but isn't then you'll need to contact 48 support for assistance.
See the links in my signature below.
