If you happen to see beer at the convenience store, could you buy it as well?
If beer is sold at the convenience store, could you get that as well?
ご質問ありがとうございます。
① "If you happen to see beer at the convenience store,"＝「もしコンビニでビールを見かけたら」
"could you buy it as well?"＝「それも買ってくれる？」
② "If beer is sold at the convenience store,"＝「もしコンビニでビールが売られていたら」
"could you get that as well?"＝「それも買ってくれる？」
ご参考に。
If they sell beer at a convenience store somewhere, can you get me some?
Since you're going out, can you stop at a convenience store to see if they have any beer?
ーIf they sell beer at a convenience store somewhere, can you get me some?
「どこかコンビニでビール売ってたら買ってきてくれる？」
somewhere で「どこか」
ーSince you're going out, can you stop at a convenience store to see if they have any beer?
「出かけるなら、コンビニに寄ってビールがあるか見てきてくれない？」
to stop at ... で「立ち寄る・寄る」
ご参考まで！
アメリカ合衆国 
カナダ 
