volvie –
One can argue that the fact remains that the stam store-owner is afraid to violate the law, and even if you find exceptions to the rule, halachically you don’t have to worry about it because in halacha we never say you need to be absolutely sure, only reasonably sure, especially with an issur d’rabbanan, and that much we are.
Also, please clarify that which you say c.y. is not a chumra but rather a lechatchila, because in this case there should be no difference.