Is this not restricted to company names deriving from personal names (or words that are perceived as such), though? For example, would you say “I bought this shirt at Target’s”?
In general, yeah, but this is sneaking into the area of language where it's all ad-hoc conventions and there aren't actually any reliable rules. I suspect, for example, that there's a sandhi component that helps predict which store names do and do not get the S added.
