. . .from a store? Or . . . at a store?

Many reasons exist to spend time every day on the Duolingo language learning site. It never grows tired or irritable at repeating a work or a phrase. It teaches the way to say something in a language, rather than teach rules of grammar—much the way a child learns. The lessons contain a variety of exercises: vocabulary drills, translations from the language into English and from English into the language, dialogues in everyday speech.

To exercise my brain I've been reviewing Japanese with the program. I'm incidentally learning contemporary Japanese—apparently much of what I say sounds old fashioned to native ears. Very occasionally I bump on a problem with the program. For example, the other day the lesson wanted students to translate the sentence "I always buy my clothes from this store."

I translated it as この店ではいつも洋服買います。(Literally and word by word "This store at always western-style clothes to buy.") 

It was marked wrong. The program wanted 洋服はいつもこの店で買います。("Western-style clothes always this store from to buy.") (The personal pronoun is understood.)

Google Translate checked both translations and Duolingo's version does mean "I always buy my clothes from this store." My version does mean "I always buy my clothes at this store."

My mistake: I automatically translated the example sentence's English into ". . . at this store." To my ear, it's unnatural in American English to say you buy clothes or food or shoes from a store. 

A Duolingo mistake? A British usage? Or the site wants students to master the form it teaches? Sometimes these questions of language usage and movement from one to the other hurt my head, but I'll never forget where I buy clothes.