The electronic shel labels use e-ink. Their refresh time is around 30 seconds. What happens if two shoppers are looking at the same product?

I would believe you if you said that electronic shelf labels use a low-power mode that checks for updates every 30 seconds, but that's not a characteristic of e-ink. (It doesn't take 30 seconds to refresh the screen of an ereader when you turn the page of an ebook.)