May I take your order?
- Yes, please. I'll have a ham sandwich and an orange juice.
- Is that all?
- Yes. How much is it all together?
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Q1: In the short dialogue, will you look at the last sentence? What about the following sentence? Can we use #1?
1. How much are they all together?
Q2: Is that all?
What about the following?
2. Are those all?
"Are those all?" is grammatically correct, but awkward. "Is that all?" is more colloquial and more common, if not exactly correct.
"How much is it..." is correct, referring to the total tab. "How much are they..." is also correct, referring to the sandwich and orange juice. The reference is implied in both, but not the same.