I went to Maru supermarket and everything was so cheap, I was shocked.
I went shopping at Maru yesterday and was surprised how inexpensive everything was.
この場合、次のように言うと良いですよ。
ーI went to Maru supermarket and everything was so cheap, I was shocked.
「マルというスーパーに行ったら、なんでもすごく安くて驚いた」
ーI went shopping at Maru yesterday and was surprised how inexpensive everything was.
「昨日マルに買い物に行ったら、なんでも安くて驚いた」
everything was so cheap や how inexpensive everything was のように表現すると自然な感じになりますよ。
ご参考まで！
カナダ 
