Noting there has not yet been a further response from Aster, I have had a look at the particular lesson and transcription referenced (with my French wife) and cannot see any glaringly obvious grammatical errors. Maybe there were some in the alternative answers, or the comment was not about this lesson. It would help to have further details as Céline notes.

As for the French phrasing,  these scripts are written by native French speakers, and the one here reflects a quite typical, polite encounter in a shop between a proprietor and customer who are not on familiar terms. It is fairly formal, but there is nothing particularly ‘unnatural ‘ about the French used.  

Of course there different ways of expressing things, as in any language, and there may be regional or other variations, but noting these doesn’t make one right and the other wrong.