If the grid is down no business will be able take digital payment. So any businesses that are open will require cash. You local grocery will probably be open for a day or two.
Hopefully, you're stocked up so you won't need anything.
items will be marked with a price, not a bar code?
if items are marked, checkout will have manual cash registers so they know how much to charge?
If the grid is down, my grocery store would not be able to open. No electricity means no transactions even with cash.