Dear Mr./Ms. Bossname,
I regret to inform you that I must resign from my position at the store.
When I was hired as a cashier, it was very clear that I was only expected to be on register. Instead, the job has morphed into part reg, part warehouse, part custodian. I am not above this labor; I am simply physically unable to perform these tasks without ongoing bodily harm to myself.
My last day will be two weeks from now. I have already secured a position more in line with my skill set as a cashier.