I will pay you back later for the purchases I would like to ask you to do for me, so could you tell me how much you would be able to cover for me?
ご質問ありがとうございます。
"I will pay you back later for the purchases"＝「後で購入した分の支払いをします」
"I would like to ask you to do for me"＝「あなたに私の代理でして欲しいのを」
"so could you tell me how much you would be able to cover for me?"＝「なので、どれほど立て替えてくれるか教えてくれますか。」
ご参考になると幸いです。
アメリカ合衆国 
