If you mean that membership auto-renewal is charging each monthly plan renewal to your registered card then I think that that's simply how it works.
I don't think that auto-renewal ever uses cash credit on your account.
To use your cash credit just buy as many membership plans as you want manually and they'll stack on your account and membership auto-renewal won't buy another one until they are used up.
If you need to contact 48 support about this then see the links in my signature below.
