It's ok if you're anxious! I think anybody would be. Working alone can be stressful, but it's even more stressful when you're by yourself and have limited experience on the job. You have every right to feel anxious.

Is there any way you can contact your manager if you're in a pickle? Like, if you don't know how to do something, can you call his number or at least someone else's number?

If need be, you can ask your manager all sorts of questions in advance and write down his answers on a piece of paper. For example, "What do I do when someone wants to buy fresh veggies but the veggies don't have a price tag on them?"