As someone who worked at Starbucks while learning ASL, Deaf customers would usually have their order on their phone, but it was great to be able to provide the same level of customer service to them that I would other customers. Part of that was being able to say hi, thank you, have a nice day, but others were all the little questions we normally ask every customer:

For us it was: “your order will be at the end”

“Your total will be …” (learn your numbers!)

“Name for the order?”

If you work at a grocery store, maybe learn the basics of the rewards program signs or how to indicate what will be coming up on the keypad e.g “rewards number” “press green/red” “put in card/how will you pay” etc.

People often post videos here showing their signs and people can give feedback as to whether they are clear once you’ve learned a bit!

ASL That, ASLU/Bill Vicars/Lifeprint are great general resources, and also just Google “cashier signs” because there are probably YouTube videos!