Advice on working a full shift alone?

Hello yall. I've been working at a 7-Eleven for about a few days now as a part time sales associate. Now I seemed to have gotten a majority of the job although there is still some stuff that I'm not too sure on. I was told I would be alone working from 2-10 P.M. on Sunday. This has gotten me extremely nervous as the few times I've been left alone I have been overwhelmed by customers. I really don't want to tell my manager that I can't do it since I have already been unavailable due to school but I don't know what else to do.

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