Strictly speaking "Парковка всегда бесплатно" is incorrect but Russian native speakers in colloquial speech often use incomplete phrases with implied parts. They feel subconsciously this phrase as "Парковка - это всегда бесплатно" or "Парковка всегда предоставляется бесплатно". So this phrase normally goes OK unless some grammar crusader starts to analyze.

BTW Short adjectives are not very popular in contemporary spoken Russian, so when this phrase is pronounced (бесплатно and бесплатна are pronounced similarly) it is most probably will be recognized as A, not as B.