Q. If someone while shopping at a frum food store places items into the shopping cart provided by the store, does he acquire the items when he picks them up or when he places them in the cart?
If not and they only become his property when he pays for them, can someone else remove them from his cart? If they then break (jars) is he responsible?
Thank you.

A. Horav Shlomo Miller’s Shlit”a opinion is that the items do not become his at the time he picks them up from the shelf or places them in the cart, since he does not have intention of acquiring them yet, as he may want to exchange them or buy something else instead.
Even though the items do not belong to him until he pays for them, it is forbidden for someone else to take them from his cart. This is similar to the case (Kidushin 59a) of someone pursuing or being engaged in the acquisition of land or the poor chasing after food, that our sages enjoined others from taking it away from them.
Rishonim disagree as to what exactly is the source and reason of the prohibition (see Rashi and Tosafot ibid.,) and if the injunction is Biblical or Rabbinic. (See Remo C,M, 237: 1, Avnei Nezer C,M, 14 et. al,). It is also comparable to the indigent cutting olives from the top of a tree, that although he did not yet make an acquisition act on them, as they fell onto the ground, it is prohibited for other to collect them because of keeping the ways of peace (Gittin 59b, Shulchan Aruch 370: 5.)
However, The Rov Shlit”a added, if the items the customer picked were to break, he would be liable for their payment as a paid shomer, since he derives benefit from them.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a