Prohibitions of Supporting Idolatry Through Commerce

Dear Rabbi, I am planning a trip to Europe and my itinerary includes a visit to a monastery with the intention of purchasing beer from their brewery. Is it permissible for me to buy beer from them?

Thank you for your question.
Monastery’s have been selling beer in Europe for centuries it was and still is a way for them to support the monks who are confined in their monasteries, and thus, this was a source of income for them. The question arises: Is buying this beer from them considered supporting idolatry?
We know that one may not benefit in any way from products that was used for idol worship this is called Is’surey Hana’ah this is based on the verse in Deuteronomy 7,26. This is especially applicable when one travels to the Far East for business, for example, and is offered fruits by the host. These could very well be fruits brought from their temples, and it would be prohibited to eat them.
However, in your case, one isn't receiving anything that was used for idol worship, so this question really goes the other way around, whether one can support a cause of idol worship.
The Gemara in Tractate Avodah Zarah (Page 12b 13a) writes that in a case where one comes to the marketplace and there are stalls owned by the followers of idol worship. There is a dispute about why one is forbidden to buy there. Reish Lakish's opinion is that it is only forbidden in a case where the stall is decorated with flowers, so when a Jew approaches, he will smell the scents of these flowers that are used for idol worship and will transgress the prohibition of not benefiting from idolatry.
However, Rabbi Yochanan's opinion is, that even if there are no idolatrous flowers at the stall, if it is just decorated with fruits as a sign that this stall belongs to idol worship followers, then it is forbidden to buy there. Since a Jew may not give financial support to the idol worship and by buying a product at this stall, one will be providing financial support through the profits to idol worship.
The Ritva writes that this is a prohibition from the Torah, and this is brought le’halacha in Yoreh De’a 149:3. However, the Shulchan Aruch adds that the reosen it is forbidden to buy at these stalls is because in those times when one bought at these stalls, there was an added tax to idol worship; that is why it is forbidden.
Meanwhile, we have learned from this halacha that it is not only forbidden to receive benefit from products that were used for idol worship but it is also forbidden to give support to their cause.
One might argue and say that when one is not paying them charity or tax, but just paying for a product, it should maybe be permitted.
Let's see another Halacha brought in the Talmud about paying for a service if it is permitted or not.
Tractate Avodah Zarah 51b states that one may use a bathhouse belonging to idol worship or enter a garden belonging to idol worship as long as it is for free. Of course, this is only permitted as long as the garden is not being used for actual idol worship and is only for decoration.
This Halacha is Me’iker Hadin. However, many tzaddikim, like the Chasam Sofer and Rabbi Yisrael Salanter (Seder Yaakov vol.2), would say that any benefit should be avoided since influences one's soul with impurity. The Chasam Sofer would even cover his ears when he heard the bells of the church ring. There was one student in Lakewood Yeshivah who started to have impure thoughts on Emunah and Ha’kadosh Baruch Hu and after a while he went to the Rosh Yeshivah to ask him why he was having such thoughts, the Rosh Yeshivah Rav Aharon Kotler observed that he had a wind-up watch so he asked him “how do you sync your watch”? The student replied, “By the church bells every morning” so the Rosh Yeshiva told him to stop doing this and you would see that the impure thoughts would leave you. The student stopped to sync his watch to the sound of the bells and the disturbing thoughts stopped occurring.
However, the above Mishnah adds, I quote, "לטובה אסורה". The Rishonim dispute about what the intention of לטובה means. Rashi writes that it means for pay, that if one has to pay to enter, it is forbidden. However, Tosafot and other Rishonim (Page 54) explain that "לטובה" means thanking him for his service as if he did you a favor, but just paying for a service is permitted.
Le’halcha we hold the opinion of Rashi and as brought in the Taz (Siman143, Se’if Katan 5 ) and one may not buy a product from such a place of idol worship.
Wishing you well.

1. Deuteronomy 7:26
2. Gemara Tractate Avodah Zarah 12b-13a
3. Ritva on Avodah Zarah
4. Shulchan Aruch, Yoreh De'a 149:3
5. Tractate Avodah Zarah 51b Rabbi Yisrael Salanter (Seder Yaakov vol.2)Taz (Siman 143, Se’if Katan 5)