It's impossible to be absolutely rational. I feel like there is so many different levels and viewpoints that there is no right answer.
Simple example:
Let's say the same pair of shoes is available in two different shops, but in one shop it's more expensive. It seem more rational to buy it in the cheaper shop. However, what if you've heard that the cheaper shop is very unethical in how it conducts the business. Is it still more rational to buy the shoes there?
And then you might also start considering this situation "in the grand scheme of things" - in the grand scheme of things does it make any difference if I buy it in shop A or B?
And at which point does it become irrational to be overthinking simple things in order to try to be rational? What if trying to always be rational is stressing you out, and turns out to be worse in the long run?
Deciding when to stop overthinking is also a rational process. Some choices truly don't matter, or not matter enough to spend time and energy on them.
If consumer ethics is important to you then it obviously warrants some deliberation, weighted by an upper bound of your potential impact. But identifying areas of meaningless choice and simply choosing randomly (and not even caring if the choice is sufficiently random) frees up a lot of mental energy.
This dilemma is addressable in a rational framework. Utilitarianism suggests you can attempt to quantify the harm done by your purchase, and decide how to discount that harm against your own self interest. You're also able to estimate whether the costs involved are probably so marginal that it's outweighed by the cost of thinking about the dilemma such that it would be best to commit to either decision rather than continue deliberating.
Yeah, for example, let's say that I can buy from ShoeCo or big, evil Amazon. But big, evil Amazon allows me to donate a portion of their proceeds to a charity of my choice, and furthermore, I am also within my rights as an individual to take the difference between ShoeCo's price and Amazon's and donate it to another cause as well.
Some will say that buying from Amazon simply perpetuates Amazon... but Amazon is so large at this point that it doesn't matter WHAT I do. So ultimately, is the world better off with my two donations from my Amazon purchase or giving my money away for the same product to ShoeCo?
If Amazon is so big that your purchase is meaningless, then the problems of the world are also so big that your donations are probably meaningless.
If your donations have some tiny bit of meaning to them, then removing a tiny bit of business from Amazon and paying your local shopkeeper probably also has meaning.
Don't think that follows automatically. My dollar - in isolation - can feed someone tomorrow, even if it doesn't feed others and they're all hungry next week. Lack of my dollar alone won't change the ethics of Amazon in the slightest, and much as the more ethical shopkeeper won't mind the extra number in his bank account it's unlikely to allow him to displace ethical companies or do anything else wonderful with it. The difference between direct, tangible outcomes and perhaps more significant outcomes which depend on a lot more other people acting in a particular way is one of the thornier questions about what's rational to prioritise. tbh when I do boycott stuff it's mostly an emotional response
(notwithstanding better objections to the original example: in practice most donors' finances aren't so tight that buying the $90 product rather than the $100 dollar one is really necessary to free up the donor funds for a worthy cause, as opposed to emotionally salve donor conscience for buying from an unworthy vendor...)
Whether a business is local means little to me if the local owner is a spendthrift who mistreats or pays his employees poorly. For example, when I was a kid, the local video shop owner would hire high school kids at minimum wage and keep them on payroll right underneath the statutory limit for certain workplace protections before firing them. Should I reward that asshole just because he's local?
Simple example:
Let's say the same pair of shoes is available in two different shops, but in one shop it's more expensive. It seem more rational to buy it in the cheaper shop. However, what if you've heard that the cheaper shop is very unethical in how it conducts the business. Is it still more rational to buy the shoes there?
And then you might also start considering this situation "in the grand scheme of things" - in the grand scheme of things does it make any difference if I buy it in shop A or B?
And at which point does it become irrational to be overthinking simple things in order to try to be rational? What if trying to always be rational is stressing you out, and turns out to be worse in the long run?