"Will that be all?" is a set phrase that people use when talking to a customer. Cashiers, hotel staff, bank tellers, etc. ask this question to find out if the customer has gotten everything that they wanted.

When someone asks "Will that be all?" you can answer like this:

A: Will that be all?

B: Yes, that's it.

A: OK. That'll be six thirty-nine.

"Will that be all?" is usually polite. Some slightly less polite versions are "Is that all?" and "Is that it?"