Two registers. (1) Polite shop farewell — equivalent of 'come again!' said by every shopkeeper as you leave. (2) Apologetic e-commerce notification — the message Taobao / Pinduoduo serves you when a flash-sale item sells out, alongside 手慢了 and 抢不到.
grammar
下次 'next time' + 再 'again' + 来 'come'. The 再 marks repetition of a prior action — 'come once more, next time'. Polite imperative; no subject is needed because it's directed at the listener.
