いくら ですか
Where the numbers and counters become a shop

One word does the whole question. The answer comes back as a number + えん.
これは いくら ですか
Unit 10 put a て-form in front for “please DO”. A noun in front means “please GIVE”.
みずを ください
で adds things up — the で of means, doing the same job it did in Unit 37.
ぜんぶで せんえん

りんごを みっつ ください。

That's Unit 57, 58 and 59 in one sentence. You can shop now.
いくら ですか
You met いくら on the ことば page. It is the whole question — no counter, no extra word. What makes this unit possible is the answer: a price is just a Unit 57 number with えん stuck on the end.
The whole question is one word you already met on the ことば page: いくら. Put the thing first, mark it with は, and finish with いくら ですか. The answer comes back as a number + えん + です — which is why Unit 57 came first.
〜を ください
You have used ください since Unit 10 — but always with a て-form in front of it, meaning please do this. Put a noun in front instead and the same word means please give me this. Everything hangs on what comes before it.
Unit 10 taught 〜て ください — “please DO this”. Put a NOUN in front of ください instead and it becomes “please GIVE me this”. The difference is entirely what sits before it: a verb in て-form asks for an action, a noun asks for the thing. おねがいします works the same way and sounds a touch more polite.
みずを ください = give me water. みずを のんで ください = drink the water. Both are polite requests, and both are correct — they just ask for different things.
かずを つける
Nothing new here — this is Unit 58's word order with ください on the end. The count still sits after the noun and its particle, never in front of the noun.
Now Unit 58 pays off. The count goes exactly where it went with any other verb — after the noun and its particle, just before ください. Nothing new to learn; two things you already have, stacked.
おみせで
Every piece of Phase Q, in one errand. Read each line, say your answer out loud, then check. The prices add up, so you can tell whether you followed it: 300 each, three of them, 900 altogether.

きょうは かいものを します。
I'm going shopping today.

いいですね。いきましょう。
Nice. Let's go.

すみません、りんごは いくら ですか。
Excuse me, how much are the apples?

ひとつ さんびゃくえん です。
300 yen each.

じゃ、りんごを みっつ ください。
Then three apples, please.

はい。ぜんぶで きゅうひゃくえん です。
Certainly. 900 yen altogether. (300 × 3)

きっては なんまい ありますか。
How many stamps do you have?

じゅうまい あります。
We have ten.
That's the phase. The wrap-up test lives on its own page now — take it whenever you're ready.