「Could I have ~?」 は、「～をいただけますか？」と丁寧に頼みたい時に使える表現です。
「extra」は「標準的なレベル、スタンダードを超えた」というイメージの単語なので、
辞書を引くと、「必要以上の、余分な、特別の、割増の、極上の、規格外の、、、」と言う色々な日本語になって出てきます。
海外の日常生活でもよく使える便利な単語のひとつです。
例）
Could I have an extra plate, please? (取り皿をもう一枚いただけますか？）
Can you give me an extra five minutes? (あと5分くれない？）
Do I need to pay any extra charge? (追加料金を支払う必要がありますか？）
ご参考になれば嬉しいです^^
こんにちは。
・Can I have some extra bags?
・Can I get some extra bags?
・Could you give me some extra bags?
上記のような言い方ができます。
can より could の方が丁寧です。some で「何枚か」、extra で「余分の」を表現できます。
ぜひ参考にしてください。
First sentence has subject I between modal verb may and main verb have forming question.
Second has subject I and adverb possibly between modal could and main verb get forming question again. Both very polite and likely to help with your request
When buying gifts you might want to give them to more friends so would need some more bags therefore you would ask for some 'extra bags' this suggests more than the one you would get with the item
possibly means that its not certain but you are asking if it is able to be done
Would you mind if I pinched a couple of extra bags please?
All three of these sentences are asking the same thing, if you could take some extra bags from the shop. They go up in politeness, sentence 1 is the most impolite (but still is polite) and sentence 3 is the most polite. With English people always being over polite, you would usually hear sentence 3 in English. "Pinched" is the same as saying take.
When you want to ask someone for extra bags when you are buying gifts for your friends, then you may ask in the following ways:
-Can I please get some extra bags?
-Could I bother you for extra bags, please?
友達にプレゼントを買うときに、余分に袋をもらいたいなら、次のように言えます。
-Can I please get some extra bags?
（袋を余分にもらえますか）
-Could I bother you for extra bags, please?
（袋を余分に頂けますか）
You want to ask for some extra bags in which you can place souvenirs for your friends. The reason for requesting the extra bags is probably not important to the person serving you. Depending on the size of the purchase they may or may not give you some extra bags without charge. However in the UK, outlets are required by law to charge a small amount for plastic bags provided.
日本 
アメリカ合衆国 
イギリス 
南アフリカ 
