I agree with księżycowy. Additionally, the indefinite article can be used when the speaker has a specific thing in mind but doesn't expect the listener to already know about it or to be able to derive it from context. So it could be that the speaker in your dialogue thinks the listener doesn't know of the shop (reasonable, since they have to ask where to get an Oyster card). It could also be the case that the shop isn't immediately visible, so that the speaker could point it out, for example.
N'hésite pas à corriger mes erreurs.