Yes. Should provide a tall stool for them to sit as long as there is no customer. A whole day of standing is terrible. Even the cashiers at the supermarkets in Europe get to sit down while serving customers.
I feel these staff should be allowed to sit as long as there is no customer to attend to. A short break from standing every now and then not only relaxes their backs or calve muscles, but also helps them to recharge and be more efficient.
Everybody seems so compassionate now, but when they see retail staff sitting down in the store, or taking a bit longer to get to their feet to serve, they will start to complain. When they see cashiers sitting down, taking their time to retrieve purchases to be scanned, those behind in the queue will complain.
Some retail staff are really trying to make ends meet and are above 55 years old. I have worked with them. By late afternoon, most are tired. So it would be ideal to let them sit in a place where they can see incoming customers and let them attend to customers when the need arises.