How often do you get something from take away? I believe most of you would since we either live alone or work until the late hours.
Here are some phrases that might come in handy along with gestures.

A. これ１つ下さい。
A. Kore hitotsu kudasai.
A. Could I have one of this?

B. のり弁当１つですね。
B. Nori Bento hitotsu desu ne.
B. One nori bento, right?

A. はい。
A. Hai.
A. Yes.

B. 少々お待ちください。
B. Shoushou omachi kudasai.
B. Please wait a while.

B. 380円になります。
B. Sanbyaku hachiju en ni narimasu.
B. That will be 380 yen.

B. 400 円ですね。おつり20円になります。
B. Yonhyaku en desune. Otsuri nijyu en ni narimasu.
B. 400 yen. 20 yen is your change.

B. ありがとうございます。
B. Arigato gozaimasu.
B. Thank you.

If you believe they have given you the wrong item, you can try this phrase.

A. 頼んだ物と違います。
A. Tanondamono to chigaimasu.
A. It's not what I ordered.

B. そうでしたか。　申し訳ありません。
B. Sou deshitaka. Moushiwake arimasen.
B. I'm sorry. You are right.

If you would like extra portion, it will be of extra cost. Depending on the place, they may only give you extra portion with the rice, but you can always ask by saying:

A. 大盛にして下さい。
A. Oomori ni shite kudasai.
A. Please give me extra. (Could you make that extra large?)