I'm not too sure how to do this, and I cannot find a redeem button or something for cards in the cash shop. Does anyone know how to redeem them, or are they not usable anymore?

The UGC link was taken down until issues with Pay-By-Cash are resolved. There's more about it on this thread. :)

Well this is a slight disappointment to me. :/ Thank you for directing me to that thread, I appreciate it.