Q. I would like to know if it is permissible to shop at thrift stores such as the Salvation Army. Stores such as these were established to provide funding for their churches, programs and charities. Since they are a religious based organization would shopping there be considered supporting their religious institution? Some yidden have expressed the sentiment that we may support them because yidden also benefit from their charities.
A. There is a disagreement between Poiskim whether the prohibition to provide benefit (mehane) to Avodah Zarah (idolatry) is of Biblical origin (Ritva, Rashba on Talmud Avoda Zarah 13a) or only Rabbinical (Kiryat Sefer, A. Z. Ch. 9). The difference is permissibility when there is doubt if this is actually happening.
Remo in Shulchan Aruch (Y.D. 143: 3) permits bestowing indirect benefit to Avodah Zarah (trough a third party) Bais Hilel (ibid.) allows if uncertainty exists if the funds given are granted to the idolatry itself or to benefit the destitute, disabled or other social services.
Minchas Itzchak (4, 84) rules that since in today's reality, most of the donated funding of religious institutions is for the support of education, the needy, social services or to sustain the clergy, you can assume that the funds provided will not go directly to idolatry. (There are also differing opinions in the Poiskim as to which religions qualify today as Avodah Zarah).
Horav Shlomo Miller's Shlit"a opinion is that considering the above, indeed there is no prohibition in buying at a Salvation Army or similar stores.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit"a