In Dickens, a clerk is someone who handles paperwork in a business office. I gather the word is still used that way in the UK.

Is it also used, as here, to mean the salesperson at a retail store?

No. We call that sort of job a shop assistant, or a sales person.

Or, here in the U.S., a clerk might be the server at a soda fountain or the person helping you in the grocery store.

We used to use the term "clerk" for the folks who check books in and out at the circulation desk. The job title has since been changed to Customer Service Associate . . . and, just in the last week, changed again to Customer Service Specialist.

I say . . . whatever.

Yes, the term "Associate" is being used more and more in the retail world, isn't it?

There is a connection here with literacy and the priesthood. The word comes from the late Latin clericus, 'priest'. We still have 'cleric' and 'clerical', of course. In the Middle Ages pretty well only the priests could read and write, so they were the ones who kept records, etc. Chaucer's Clerk was a priest. Later on people were employed in record-keeping jobs who were not ordained, but the job title remained.

Quite why a soda-jerk at a drugstore should be called a clerk is a mystery to me, as I wouldn't have thought that literacy was a job requirement.