So you mean that if this percentage is under a certain limit then it’s ok, but if it was notably higher then it would be immoral to buy at Starbucks? But that would mean (assuming that Starbucks was indeed funding PP) that you’re allowed to buy only during some years, until your total contribution start to be too high.
Also, there is the fact that, even if your contribution is tiny in itself, together with other peoples contributions it amounts to a big sum (in the same way that your single vote in an election is not much, but it still carries moral weight because you know that together with the other votes it will have important consequences).
That’s also what I thought. However, I have a doubt because of the principle of double effect. According to this principle, even if an act is not in itself immoral, you don’t have the right to do it if you know it will have a bad consequence, unless you know it will also have a proportionally equal good consequence. However, in cases where buying at a company results in a reasonably large amount of money going to immoral causes (whether you consider your own money alone, or together with other peoples contributions), then there is hardly a proportionally good consequence (except when it’s a product you absolutely need, like basic clothes or food).