All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad  is His slave and Messenger.
  is His slave and Messenger.
It was reported in the Sunnah that it is prohibited for a woman to be in seclusion with a non-Mahram man. A warning against that is mentioned in the Hadith reported by Ahmad and At-Tirmidhi  and narrated by ‘Umar
  and narrated by ‘Umar  that the Prophet
  that the Prophet  said: “There is absolutely no man who stays in seclusion with a woman except that the devil is their third companion.”
  said: “There is absolutely no man who stays in seclusion with a woman except that the devil is their third companion.”
Scholars have clarified the rules for the forbidden seclusion and that it is a man and a non-Mahram woman staying alone in a place where a third person cannot enter, so there will be suspicion, and one is not safe from temptation.
Based on that, your work in the grocery store is permissible even if a woman enters alone to buy from you because the grocery store is considered one of the public places people frequently visit in a way that a person can enter at any time, thus negating the forbidden seclusion.
Allah knows best.