If this is a medical issue they will accommodate you with a chair or with less hours per shift if that is something you want.
Of this is not a medical issue, i dont want to sound offensive and i dont mean it that way as i have seen this a few times but every other job at spot is also standing for the most part, and you just dont want to stand and cashier for 7 hours a day, it is best to be very direct with your leadership team and ask to be trained in other parts of the store and to maybe do a couple shifts a week somewhere else.